Having a font/span tag with applied inline font-size style cannot be formatted by FontSize tool. This is a possible workaround that removes font-size inline style when size attribute is to be added: <telerik:RadEditor runat="server" ID="RadEditor1" OnClientCommandExecuting="OnClientCommandExecuting"> <Tools> <telerik:EditorToolGroup> <telerik:EditorTool Name="FontSize" /> </telerik:EditorToolGroup> </Tools> <Content> <p><span style="font-size:15px;">text</span></p> </Content> </telerik:RadEditor> <script> function OnClientCommandExecuting(editor, args) { var command = args.get_commandName(); if (command === "FontSize") { $telerik.$(editor.getSelectedElement()).css("font-size", ""); } } </script>