Use VDividedBox
<?xml version="1.0" encoding="utf-8"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"> <mx:Panel layout="horizontal" width="100%" height="100%" title="this is a test"> <mx:VDividedBox width="200" height="100%"> <mx:List width="100%" height="200"> </mx:List> <mx:List width="100%"> </mx:List> </mx:VDividedBox> </mx:Panel> </mx:Application>