>If this code is in the Form.Activate() you need to take steps to ensure that is only runs once - otherwise it will attempt to add the nodes to the tree again.
>
>Yes first treview works fine. Every thing shows ok on its place. Second time when FORM get ACTIVATE evnets then error message appears
>
>What is wrong?

As above - you're attempting to load the tree each time Activate() fires. Can you put this code in the Init() instead - or set a flag the first time Activate fires and skip the operation on subsequent events.