Set a Button control's fontSize property as font-size or fontSize in an declaration
<?xml version="1.0"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"> <mx:Style> .myFontStyle { fontSize: 15; } .myOtherFontStyle { font-size:15; } </mx:Style> <mx:Button id="myButton" styleName="myFontStyle" label="Click Here" /> <mx:Button id="myButton2" styleName="myOtherFontStyle" label="Click Here" /> <mx:Button id="myButton3" fontSize="15" label="Click Here" /> </mx:Application>