Such a scenario is reproducible when RadEditor is used in Template container and the control is AJAX-enabled. For example, in an EditTemplate or InsertTemplate. To workaround this bug, you need to assure that the base CSS resource is loaded before RadEditor. To do so, add manually the resource in the head element of the page: <head runat="server"> <title></title> <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server"> <link href='<%= Page.ClientScript.GetWebResourceUrl(typeof(RadEditor), "Telerik.Web.UI.Skins.EditorLite.css") %>' rel="stylesheet" type="text/css" /> </telerik:RadCodeBlock> </head>