Hi Thi Han,
For this scenario you can use the Load-On-Demand hierarchy mode. More information how to use and implement his behavior you can find in our online documentation.
I hope this information is useful. Let me know if you need further assistance.
All the best,
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.