I have work around my problem.
When I spend my time for find out solution , I recognize that when event of client javascript excute, then event setfocus of ScriptManager from code behind will be effect.
So that I try with below code, and It's work at both IE7 & IE8.
It's look like a double focus but you can replace the focus event
by
RegisterStartupScript
with another event some kind like show alert messenge, focus other control, then finally
ScriptManager1.SetFocus
will be work.