diff --git a/Cargo.lock b/Cargo.lock
index 29ff6ddb2a..f26d6e7e2f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -445,6 +445,7 @@ dependencies = [
  "deno_core",
  "deno_typescript",
  "dirs",
+ "dissimilar",
  "dlopen",
  "dprint-plugin-typescript",
  "futures 0.3.5",
@@ -553,6 +554,12 @@ dependencies = [
  "winapi 0.3.8",
 ]
 
+[[package]]
+name = "dissimilar"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39de161cd2ebbd6e5783db53a82a47b6a47dcfef754130839603561745528b94"
+
 [[package]]
 name = "dlopen"
 version = "0.1.8"
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index d6176bcf60..81eb9d7fbf 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -28,6 +28,7 @@ bytes = "0.5.4"
 byteorder = "1.3.4"
 clap = "2.33.1"
 dirs = "2.0.2"
+dissimilar = "1.0"
 dlopen = "0.1.8"
 dprint-plugin-typescript = "0.19.2"
 futures = { version = "0.3.5", features = ["compat", "io-compat"] }
diff --git a/cli/colors.rs b/cli/colors.rs
index 764704a5b8..b9a5a7353e 100644
--- a/cli/colors.rs
+++ b/cli/colors.rs
@@ -52,6 +52,12 @@ pub fn red_bold(s: String) -> impl fmt::Display {
   style(&s, style_spec)
 }
 
+pub fn green_bold(s: String) -> impl fmt::Display {
+  let mut style_spec = ColorSpec::new();
+  style_spec.set_fg(Some(Ansi256(10))).set_bold(true);
+  style(&s, style_spec)
+}
+
 pub fn italic_bold(s: String) -> impl fmt::Display {
   let mut style_spec = ColorSpec::new();
   style_spec.set_bold(true).set_italic(true);
@@ -64,6 +70,18 @@ pub fn black_on_white(s: String) -> impl fmt::Display {
   style(&s, style_spec)
 }
 
+pub fn white_on_red(s: String) -> impl fmt::Display {
+  let mut style_spec = ColorSpec::new();
+  style_spec.set_bg(Some(Red)).set_fg(Some(White));
+  style(&s, style_spec)
+}
+
+pub fn white_on_green(s: String) -> impl fmt::Display {
+  let mut style_spec = ColorSpec::new();
+  style_spec.set_bg(Some(Ansi256(10))).set_fg(Some(White));
+  style(&s, style_spec)
+}
+
 pub fn yellow(s: String) -> impl fmt::Display {
   let mut style_spec = ColorSpec::new();
   style_spec.set_fg(Some(Ansi256(11)));
diff --git a/cli/diff.rs b/cli/diff.rs
new file mode 100644
index 0000000000..e6d7fa4bb0
--- /dev/null
+++ b/cli/diff.rs
@@ -0,0 +1,171 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+use crate::colors;
+use dissimilar::{diff as difference, Chunk};
+use std::fmt;
+use std::fmt::Write;
+
+fn fmt_add() -> String {
+  format!("{}", colors::green_bold("+".to_string()))
+}
+
+fn fmt_add_text(x: String) -> String {
+  format!("{}", colors::green(x))
+}
+
+fn fmt_add_text_highlight(x: String) -> String {
+  format!("{}", colors::white_on_green(x))
+}
+
+fn fmt_rem() -> String {
+  format!("{}", colors::red_bold("-".to_string()))
+}
+
+fn fmt_rem_text(x: String) -> String {
+  format!("{}", colors::red(x))
+}
+
+fn fmt_rem_text_highlight(x: String) -> String {
+  format!("{}", colors::white_on_red(x))
+}
+
+fn write_line_diff(
+  diff: &mut String,
+  orig_line: &mut usize,
+  edit_line: &mut usize,
+  line_number_width: usize,
+  orig: &mut String,
+  edit: &mut String,
+) -> fmt::Result {
+  let split = orig.split('\n').enumerate();
+  for (i, s) in split {
+    write!(
+      diff,
+      "{:0width$}{} ",
+      *orig_line + i,
+      colors::gray("|".to_string()),
+      width = line_number_width
+    )?;
+    write!(diff, "{}", fmt_rem())?;
+    write!(diff, "{}", s)?;
+    writeln!(diff)?;
+  }
+
+  let split = edit.split('\n').enumerate();
+  for (i, s) in split {
+    write!(
+      diff,
+      "{:0width$}{} ",
+      *edit_line + i,
+      colors::gray("|".to_string()),
+      width = line_number_width
+    )?;
+    write!(diff, "{}", fmt_add())?;
+    write!(diff, "{}", s)?;
+    writeln!(diff)?;
+  }
+
+  *orig_line += orig.split('\n').count();
+  *edit_line += edit.split('\n').count();
+
+  orig.clear();
+  edit.clear();
+
+  Ok(())
+}
+
+/// Print diff of the same file_path, before and after formatting.
+///
+/// Diff format is loosely based on Github diff formatting.
+pub fn diff(orig_text: &str, edit_text: &str) -> Result<String, fmt::Error> {
+  let lines = edit_text.split('\n').count();
+  let line_number_width = lines.to_string().chars().count();
+
+  let mut diff = String::new();
+
+  let mut text1 = orig_text.to_string();
+  let mut text2 = edit_text.to_string();
+
+  if !text1.ends_with('\n') {
+    writeln!(text1)?;
+  }
+  if !text2.ends_with('\n') {
+    writeln!(text2)?;
+  }
+
+  let mut orig_line: usize = 1;
+  let mut edit_line: usize = 1;
+  let mut orig: String = String::new();
+  let mut edit: String = String::new();
+  let mut changes = false;
+
+  let chunks = difference(&text1, &text2);
+  for chunk in chunks {
+    match chunk {
+      Chunk::Delete(s) => {
+        let split = s.split('\n').enumerate();
+        for (i, s) in split {
+          if i > 0 {
+            orig.push_str("\n");
+          }
+          orig.push_str(&fmt_rem_text_highlight(s.to_string()));
+        }
+        changes = true
+      }
+      Chunk::Insert(s) => {
+        let split = s.split('\n').enumerate();
+        for (i, s) in split {
+          if i > 0 {
+            edit.push_str("\n");
+          }
+          edit.push_str(&fmt_add_text_highlight(s.to_string()));
+        }
+        changes = true
+      }
+      Chunk::Equal(s) => {
+        let split = s.split('\n').enumerate();
+        for (i, s) in split {
+          if i > 0 {
+            if changes {
+              write_line_diff(
+                &mut diff,
+                &mut orig_line,
+                &mut edit_line,
+                line_number_width,
+                &mut orig,
+                &mut edit,
+              )?;
+              changes = false
+            } else {
+              orig.clear();
+              edit.clear();
+              orig_line += 1;
+              edit_line += 1;
+            }
+          }
+          orig.push_str(&fmt_rem_text(s.to_string()));
+          edit.push_str(&fmt_add_text(s.to_string()));
+        }
+      }
+    }
+  }
+  Ok(diff)
+}
+
+#[test]
+fn test_diff() {
+  let simple_console_log_unfmt = "console.log('Hello World')";
+  let simple_console_log_fmt = "console.log(\"Hello World\");";
+  assert_eq!(
+    colors::strip_ansi_codes(
+      &diff(simple_console_log_unfmt, simple_console_log_fmt).unwrap()
+    ),
+    "1| -console.log('Hello World')\n1| +console.log(\"Hello World\");\n"
+  );
+
+  let line_number_unfmt = "\n\n\n\nconsole.log(\n'Hello World'\n)";
+  let line_number_fmt = "console.log(\n\"Hello World\"\n);";
+  assert_eq!(
+    colors::strip_ansi_codes(&diff(line_number_unfmt, line_number_fmt).unwrap()),
+    "1| -\n2| -\n3| -\n4| -\n5| -console.log(\n1| +console.log(\n6| -'Hello World'\n2| +\"Hello World\"\n7| -)\n3| +);\n"
+  )
+}
diff --git a/cli/fmt.rs b/cli/fmt.rs
index 5d1becffa5..0d7a909bc0 100644
--- a/cli/fmt.rs
+++ b/cli/fmt.rs
@@ -7,6 +7,8 @@
 //! the future it can be easily extended to provide
 //! the same functions as ops available in JS runtime.
 
+use crate::colors;
+use crate::diff::diff;
 use crate::fs::files_in_subtree;
 use crate::op_error::OpError;
 use deno_core::ErrBox;
@@ -63,7 +65,9 @@ async fn check_source_files(
 ) -> Result<(), ErrBox> {
   let not_formatted_files_count = Arc::new(AtomicUsize::new(0));
   let formatter = Arc::new(dprint::Formatter::new(config));
-  let output_lock = Arc::new(Mutex::new(0)); // prevent threads outputting at the same time
+
+  // prevent threads outputting at the same time
+  let output_lock = Arc::new(Mutex::new(0));
 
   run_parallelized(paths, {
     let not_formatted_files_count = not_formatted_files_count.clone();
@@ -74,6 +78,25 @@ async fn check_source_files(
         Ok(formatted_text) => {
           if formatted_text != file_text {
             not_formatted_files_count.fetch_add(1, Ordering::SeqCst);
+            let _g = output_lock.lock().unwrap();
+            match diff(&file_text, &formatted_text) {
+              Ok(diff) => {
+                println!();
+                println!(
+                  "{} {}:",
+                  colors::bold("from".to_string()),
+                  file_path.display().to_string()
+                );
+                println!("{}", diff);
+              }
+              Err(e) => {
+                eprintln!(
+                  "Error generating diff: {}",
+                  file_path.to_string_lossy()
+                );
+                eprintln!("   {}", e);
+              }
+            }
           }
         }
         Err(e) => {
diff --git a/cli/main.rs b/cli/main.rs
index 7da0e2df36..cf819ce5f8 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -1,6 +1,7 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 #![deny(warnings)]
 
+extern crate dissimilar;
 #[macro_use]
 extern crate lazy_static;
 #[macro_use]
@@ -25,6 +26,7 @@ mod checksum;
 pub mod colors;
 pub mod deno_dir;
 pub mod diagnostics;
+mod diff;
 mod disk_cache;
 mod doc;
 mod file_fetcher;