Hello Loy Chan,
Thank you for getting back to us.
Actually, RadComboBox behavior is consistent to the one of Microsoft ComboBox. When you type some text in RadComboBox or ComboBox, and if an item with such text exists, it will become the selected item (and the SelectedIndexChanged event will be fired) when you Tab off the control. I am attaching a sample project to demonstrate this.
If you have additional questions, feel free to contact me.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.