When initially loading data form the web service the ClientTemplates are applied to the control/items normally. On PostBack of the page, however, the ClientTemplate gets lost. This is due to the fact that on postback the items are recreated on the server and are no longer loaded through web service. In order to preserve the templated look, a server side template needs to be applied. So in order to have consistent look when using ClientTemplates and PostBacks, you have to include Server template with the same layout as the ClientTemplate, that is applied to nodes that come from the server.
Keep in mind that on PageLoad the DataBind() event needs to be called as well ( RadTreeView1.DataBind(); ).
<telerik:RadTreeView RenderMode="Lightweight" ID="RadTreeView1" runat="server" > <ClientNodeTemplate> Template Text: #= Text # </ClientNodeTemplate> <NodeTemplate> Template Text : <%# DataBinder.Eval(Container, "Text") %> </NodeTemplate> <WebServiceSettings Path="TreeViewWebServicePageMethod.aspx" Method="GetChildren" /> </telerik:RadTreeView>