diff --git a/README.md b/README.md
index a37ef0a..61ed291 100644
--- a/README.md
+++ b/README.md
@@ -158,6 +158,19 @@ refactor(*): clean up
 
 The above commit causes `patch` upgrade to the all packages.
 
+## Unstable updates
+
+You can mark the change only affects the unstable part of the package by using
+`scope/unstable` or `unstable/scope`.
+
+```
+feat(crypto/unstable): a new unstable feature
+BREAKING(crypto/unstable): breaking change to unstable feature
+```
+
+If this notation is used, the effect of the commit becomes `patch` no matter
+what commit type is used.
+
 # License
 
 MIT
diff --git a/util.ts b/util.ts
index 25660dd..3c46fd6 100644
--- a/util.ts
+++ b/util.ts
@@ -88,6 +88,7 @@ export type VersionUpdateResult = {
 };
 
 const RE_DEFAULT_PATTERN = /^([^:()]+)(?:\((.+)\))?: (.*)$/;
+const REGEXP_UNSTABLE_SCOPE = /^(unstable\/(.+)|(.+)\/unstable)$/;
 
 // Defines the version bump for each tag.
 const TAG_TO_VERSION: Record<string, "major" | "minor" | "patch"> = {
@@ -153,7 +154,20 @@ export function defaultParseCommitMessage(
       reason: `Unknown commit tag: ${tag}.`,
     };
   }
-  return modules.map((module) => ({ module, tag, version, commit }));
+  return modules.map((module) => {
+    const matchUnstable = REGEXP_UNSTABLE_SCOPE.exec(module);
+    if (matchUnstable) {
+      // 'scope' is in the form of unstable/foo or foo/unstable
+      // In this case all changes are considered as patch
+      return {
+        module: matchUnstable[2] || matchUnstable[3],
+        tag,
+        commit,
+        version: "patch",
+      };
+    }
+    return ({ module, tag, version, commit });
+  });
 }
 
 export function summarizeVersionBumpsByModule(
diff --git a/util_test.ts b/util_test.ts
index 303494a..12472fd 100644
--- a/util_test.ts
+++ b/util_test.ts
@@ -205,6 +205,35 @@ Deno.test("defaultParseCommitMessage()", () => {
       },
     },
   ]);
+
+  assertEquals(parse("feat(foo/unstable): a new unstable feature", modules), [
+    {
+      module: "foo",
+      tag: "feat",
+      version: "patch",
+      commit: {
+        subject: "feat(foo/unstable): a new unstable feature",
+        body: "",
+        hash,
+      },
+    },
+  ]);
+
+  assertEquals(
+    parse("BREAKING(unstable/foo): break some unstable feature", modules),
+    [
+      {
+        module: "foo",
+        tag: "BREAKING",
+        version: "patch",
+        commit: {
+          subject: "BREAKING(unstable/foo): break some unstable feature",
+          body: "",
+          hash,
+        },
+      },
+    ],
+  );
 });
 
 Deno.test("checkModuleName()", () => {