RadComboBox supports client-side filtering - end users can perform fast search among all items without going to the server.
The Filter property has three values:

• None - this is the default value. No filtering is done.
• Contains - will return all items which text contains the search text anywhere
• StartsWith - will return all items which text starts with the search text

To enable the filtering mechanism of the combobox you need to set the Filter property to either Contains or StartsWith

code of combo box

 telerik:Radcombobox runat="server"
                                             ID="ddlSystem"
                                             DataTextField="System"
                                             DataValueField="SystemID"
                                             HighlightTemplatedItems="true"
                                             Height="190px"
                                             Width="220px"
                                             DropDownWidth="420px"
                                             DataSourceID="sqlSystem"
                                             EnableItemCaching="True"
                                             EnableLoadOnDemand="True"
                                             emptymessage="Please select one..."
                                             allowcustomtext="True" MarkFirstMatch="True"
                                               >/telerik:RadComboBox>

code of datasource

  asp:LinqDataSource ID="sqlSystem" runat="server" ContextTypeName="DataClassesDataContext"
                                        TableName="tbl_LU_Systems" AutoPage="False"
                                        OrderBy="System">
  /asp:LinqDataSource>                                     

set filter

 ddlSystem.Filter = (RadComboBoxFilter)Convert.ToInt32(2);