Hello Phuoc Tran Ngoc,
Thank you for contacting us.
RadTextBox does not offer formatting out of the box, but your scenario can be achieved by a few lines of code. One possible solution I came up with is using the TextChanged event of the RadTextBox and manually formatting the text. Here is a code snippet:
I hope you find this useful. If you need further assistance, I would be glad to help.
Kind regards,
Ivan Petrov
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.