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