mActionBar.setHomeButtonEnabled(true);
mActionBar.setDisplayHomeAsUpEnabled(true);

两者缺一不可。

public boolean onOptionsItemSelected(MenuItem item) {
		switch (item.getItemId()) {
		case android.R.id.home:
			onBackPressed();
			break;
		}
		return true;
	}

android_HomeAsUp返回按钮