i use RequiredFieldValidator to validate radtextbox,SetFocusOnError="true" telerik version:2012Q3 first ,enter " ",validate event fire, display “notnull” sencond,enter “123” ,radtextbox not set value,validate event not fire asp.net textbox is ok code: <telerik:RadTextBox ID="txtAppId" runat="server" Width="300" AutoPostBack="false"> </telerik:RadTextBox> <%--<asp:TextBox ID="txtAppId" runat="server" Width="300" AutoPostBack="false"> </asp:TextBox>--%> <asp:RequiredFieldValidator ID="TextBoxRequiredFieldValidator" runat="server" Display="Dynamic" ControlToValidate="txtAppId" Text="NOT NULL" ErrorMessage="NOT NULL" SetFocusOnError="true"> </asp:RequiredFieldValidator>