diff --git a/README.md b/README.md
index 4799fdf4..e2a825c3 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ A [shadowsocks](http://shadowsocks.org) client for Android, written in Scala.
 ### BUILD
 
 * Set environment variable `ANDROID_HOME` to `/path/to/android-sdk`
-* Set environment variable `ANDROID_NDK_HOME` to `/path/to/android-ndk`
+* (optional) Set environment variable `ANDROID_NDK_HOME` to `/path/to/android-ndk` (default: `$ANDROID_HOME/ndk-bundle`)
 * Create your key following the instructions at https://developer.android.com/studio/publish/app-signing.html
 * Create `mobile/local.properties` from `mobile/local.properties.example` with your own key information
 * Invoke the building like this