I found the answer there is a property called IsLoadOnDemandEnabled and set this property to false on ItemPrepared event.
<telerik:RadTreeView x:Name="radTreeView"
IsExpandOnSingleClickEnabled="True"
IsLoadOnDemandEnabled="true"
LoadOnDemand="RadTreeView_LoadOnDemand"
ItemPrepared="radTreeView_ItemPrepared"
ItemsSource="{Binding TreeViewSource,Mode=OneWay}"
ItemTemplate="{StaticResource ParentTemplate}"
/>
and in the xaml.cs
private void radTreeView_ItemPrepared(object sender, RadTreeViewItemPreparedEventArgs e)
{
// get a reference to the item that has been selected
RadTreeViewItem preparedItem = e.PreparedItem as RadTreeViewItem;
preparedItem.IsLoadOnDemandEnabled = false;
}
for reference
http://www.telerik.com/help/wpf/radtreeview-features-load-on-demand.html