First of all,u take one TextBox1 control then used to FilterTextboxExtender under AjaxControl.Now go to properties of FilterTextboxExtender n choose the TargetID properties and write the Textbox1 Id(Bydefault ID will be Textbox1)there. Now go to TextBox1 Properties there will be auto create the FilterTextbox Property there willbe one option ValidChars.So here u will put this following 0123456789 characters there.