android:orientation="horizontal" does not function

android:orientation="horizontal" does not function

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="horizontal" >
When I start this activity, its orientation just inherits the current
orientation, no change is made, did I miss something anywhere?