Common scenario is to use PrimaryIconCssClass and SecondaryIconCssClass to decorate custom icons. Common situation is using bootstrap's classes for the glyphicons. In order to workaround this you can use custom CSS rules and define the expected content properties for the before elements. This is an example with bootstrap icons (http://glyphicons.bootstrapcheatsheets.com/). <style> .rbPrimaryIcon.glyphicon-filter:before, .rbSecondaryIcon.glyphicon-filter:before { content: "\e138"; } </style> <telerik:RadButton ID="RadButton2" runat="server" Text="Filter" ButtonType="SkinnedButton" Icon-PrimaryIconCssClass="glyphicon glyphicon-filter" > </telerik:RadButton>