Hello Seunggyun,

Thank you for posting in Infragistics forum.

You should be able to expand a node on load by calling node’s set_expanded() method at WebDataTree's Initialize event like below.

Javascript
------------------------------
function WebDataTree1_Initialize(sender, eventArgs)
{
    if('<%= Page.IsPostBack%>' != 'True')
    {
        sender.getNode(0).set_expanded(true);
        sender.getNode(0).get_childNode(0).set_expanded(true);
    }  
}
------------------------------
aspx
------------------------------
        <ig:WebDataTree ID="WebDataTree1">
…..
            <ClientEvents Initialize="WebDataTree1_Initialize" />
        </ig:WebDataTree>
------------------------------

I have attached a sample for your reference.

Please let me know if I may be of further assistance.

7853.WebApplication1.zip