diff --git a/editor/decoration.js b/editor/decoration.js
index 649eeb0..6d8d038 100644
--- a/editor/decoration.js
+++ b/editor/decoration.js
@@ -3,13 +3,15 @@ import {outputLinesField} from "./outputLines";
 import {RangeSetBuilder} from "@codemirror/state";
 
 const highlight = Decoration.line({attributes: {class: "cm-output-line"}});
+const errorHighlight = Decoration.line({attributes: {class: "cm-output-line cm-error-line"}});
 // const linePrefix = Decoration.mark({attributes: {class: "cm-output-line-prefix"}});
 // const lineContent = Decoration.mark({attributes: {class: "cm-output-line-content"}});
 
 function createWidgets(lines) {
   const builder = new RangeSetBuilder();
-  for (const {from, to} of lines) {
-    builder.add(from, from, highlight);
+  for (const {from, type} of lines) {
+    if (type === "output") builder.add(from, from, highlight);
+    else if (type === "error") builder.add(from, from, errorHighlight);
     // builder.add(from, from + 3, linePrefix);
     // builder.add(from + 4, to, lineContent);
   }
diff --git a/editor/index.css b/editor/index.css
index dca6b9c..29cdb12 100644
--- a/editor/index.css
+++ b/editor/index.css
@@ -143,6 +143,17 @@
     /*background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='4' height='4' viewBox='0 0 4 4'%3E%3Cpath fill='%237d7d7d' fill-opacity='0.4' d='M1 3h1v1H1V3zm2-2h1v1H3V1z'%3E%3C/path%3E%3C/svg%3E");*/
   }
 
+  /* We can't see background color, which is conflict with selection background color.
+   * So we use svg pattern to simulate the background color.
+   */
+  .cm-output-line.cm-error-line {
+    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='4' height='4' viewBox='0 0 4 4' %3E%3Cpath fill='%23ff0000' fill-opacity='0.4' d='M1 3h1v1H1V3zm2-2h1v1H3V1z'%3E%3C/path%3E%3C/svg%3E");
+  }
+
+  .cm-output-line.cm-error-line span {
+    color: #c83f30 !important;
+  }
+
   .cm-doc-tag span {
     color: #cf222e;
   }
diff --git a/editor/outputLines.js b/editor/outputLines.js
index b50172e..2e99584 100644
--- a/editor/outputLines.js
+++ b/editor/outputLines.js
@@ -1,9 +1,11 @@
 import {syntaxTree} from "@codemirror/language";
 import {StateField} from "@codemirror/state";
-import {OUTPUT_MARK} from "../runtime/constant.js";
+import {OUTPUT_MARK, ERROR_MARK} from "../runtime/constant.js";
 
 const OUTPUT_MARK_CODE_POINT = OUTPUT_MARK.codePointAt(0);
 
+const ERROR_MARK_CODE_POINT = ERROR_MARK.codePointAt(0);
+
 /** @type {StateField<{number: number, from: number, to: number}[]>} */
 export const outputLinesField = StateField.define({
   create(state) {
@@ -25,11 +27,21 @@ function computeLineNumbers(state) {
       if (node.name === "LineComment" && node.node.parent.name === "Script") {
         // Check if the line comment covers the entire line.
         const line = state.doc.lineAt(node.from);
-        if (line.from === node.from && line.to === node.to && line.text.codePointAt(2) === OUTPUT_MARK_CODE_POINT) {
+        if (line.from !== node.from || line.to !== node.to) return;
+        if (line.text.codePointAt(2) === OUTPUT_MARK_CODE_POINT) {
+          lineNumbers.push({
+            number: line.number,
+            from: line.from,
+            to: line.to,
+            type: "output",
+          });
+        }
+        if (line.text.codePointAt(2) === ERROR_MARK_CODE_POINT) {
           lineNumbers.push({
             number: line.number,
             from: line.from,
             to: line.to,
+            type: "error",
           });
         }
       }
diff --git a/runtime/constant.js b/runtime/constant.js
index ea11373..ea75326 100644
--- a/runtime/constant.js
+++ b/runtime/constant.js
@@ -1 +1,3 @@
 export const OUTPUT_MARK = "➜";
+
+export const ERROR_MARK = "✗";
diff --git a/runtime/index.js b/runtime/index.js
index 2857e7a..ec5ae60 100644
--- a/runtime/index.js
+++ b/runtime/index.js
@@ -5,9 +5,11 @@ import {parse} from "acorn";
 import {group} from "d3-array";
 import {dispatch as d3Dispatch} from "d3-dispatch";
 import * as stdlib from "./stdlib.js";
-import {OUTPUT_MARK} from "./constant.js";
+import {OUTPUT_MARK, ERROR_MARK} from "./constant.js";
 
-const PREFIX = `//${OUTPUT_MARK}`;
+const OUTPUT_PREFIX = `//${OUTPUT_MARK}`;
+
+const ERROR_PREFIX = `//${ERROR_MARK}`;
 
 const BUILTINS = {
   recho: () => stdlib,
@@ -17,6 +19,10 @@ function uid() {
   return Math.random().toString(36).substring(2, 15) + Math.random().toString(36).substring(2, 15);
 }
 
+function isError(value) {
+  return value instanceof Error;
+}
+
 function safeEval(code, inputs) {
   const body = `const foo = ${code}; return foo(${inputs.join(",")})`;
   const fn = new Function(...inputs, body);
@@ -46,10 +52,18 @@ function inspect(value, {limit = 200, quote = "double", indent = null} = {}) {
   return string;
 }
 
-function format(value, options) {
+function format(value, options, prefix) {
   const string = inspect(value, options);
   const lines = string.split("\n");
-  return lines.map((line) => `${PREFIX} ${line}`).join("\n");
+  return lines.map((line) => `${prefix} ${line}`).join("\n");
+}
+
+function formatOutput(value, options) {
+  return format(value, options, OUTPUT_PREFIX);
+}
+
+function formatError(value, options) {
+  return format(value, options, ERROR_PREFIX);
 }
 
 export function createRuntime(initialCode) {
@@ -72,7 +86,8 @@ export function createRuntime(initialCode) {
       const start = node.start;
       const {values} = node.state;
       if (values.length) {
-        const output = values.map(({value, options}) => format(value, options)).join("\n") + "\n";
+        const f = (v, o) => (isError(v) ? formatError(v, o) : formatOutput(v, o));
+        const output = values.map(({value, options}) => f(value, options)).join("\n") + "\n";
         changes.push({from: start, insert: output});
       }
     }
@@ -127,7 +142,7 @@ export function createRuntime(initialCode) {
     } catch (error) {
       console.error(error);
       const changes = removeChanges(code);
-      const errorMsg = format(error) + "\n";
+      const errorMsg = formatError(error) + "\n";
       changes.push({from: 0, insert: errorMsg});
       dispatch(changes);
       return null;
@@ -140,7 +155,7 @@ export function createRuntime(initialCode) {
     } catch (error) {
       console.error(error);
       const changes = removeChanges(code);
-      const errorMsg = format(error) + "\n";
+      const errorMsg = formatError(error) + "\n";
       changes.push({from: 0, insert: errorMsg});
       dispatch(changes);
       return null;
@@ -153,7 +168,7 @@ export function createRuntime(initialCode) {
     const oldOutputs = code
       .split("\n")
       .map((l, i) => [l, i])
-      .filter(([l]) => l.startsWith(PREFIX))
+      .filter(([l]) => l.startsWith(OUTPUT_PREFIX) || l.startsWith(ERROR_PREFIX))
       .map(([_, i]) => i);
 
     const lineOf = (i) => {
diff --git a/test/js/runtime-error.js b/test/js/runtime-error.js
index 657bfac..491559d 100644
--- a/test/js/runtime-error.js
+++ b/test/js/runtime-error.js
@@ -1 +1,5 @@
-export const runtimeError = `add(1, 2);`;
+export const runtimeError = `const add = (a, b) => a + b;
+
+ad(1, 2);
+
+echo(add(1, 2));`;
diff --git a/test/js/syntax-error.js b/test/js/syntax-error.js
index 5b7813d..cd21f8d 100644
--- a/test/js/syntax-error.js
+++ b/test/js/syntax-error.js
@@ -1 +1,5 @@
-export const syntaxError = `function add();`;
+export const syntaxError = `const a = 1;
+
+function add();
+
+echo(1 + 2);`;
diff --git a/test/output/runtimeError.js b/test/output/runtimeError.js
index 0e660d4..88758ee 100644
--- a/test/output/runtimeError.js
+++ b/test/output/runtimeError.js
@@ -1,2 +1,7 @@
-//➜ { [RuntimeError: add is not defined] input: "add" }
-add(1, 2);
\ No newline at end of file
+const add = (a, b) => a + b;
+
+//✗ { [RuntimeError: ad is not defined] input: "ad" }
+ad(1, 2);
+
+//➜ 3
+echo(add(1, 2));
\ No newline at end of file
diff --git a/test/output/syntaxError.js b/test/output/syntaxError.js
index c0f49ba..083805b 100644
--- a/test/output/syntaxError.js
+++ b/test/output/syntaxError.js
@@ -1,2 +1,6 @@
-//➜ { [SyntaxError: Unexpected token (1:14)] pos: 14, loc: Position { line: 1, column: 14 }, raisedAt: 15 }
-function add();
\ No newline at end of file
+//✗ { [SyntaxError: Unexpected token (3:14)] pos: 28, loc: Position { line: 3, column: 14 }, raisedAt: 29 }
+const a = 1;
+
+function add();
+
+echo(1 + 2);
\ No newline at end of file
diff --git a/test/output/syntaxError2.js b/test/output/syntaxError2.js
index b176b97..5c87923 100644
--- a/test/output/syntaxError2.js
+++ b/test/output/syntaxError2.js
@@ -1,2 +1,2 @@
-//➜ [SyntaxError: Assignment to external variable 'a' (1:0)]
+//✗ [SyntaxError: Assignment to external variable 'a' (1:0)]
 let a = 1; a = 2;
\ No newline at end of file