Hi,
I work for a company in Kansas. My annual income is 58k$ and my sign on bonus is 7000$. But I received a sign on bonus check of 4900$. The federal tax withheld was 1750$ and state tax 350$. I am on my my OPT still(F1 status). Why has so much amount been withheld on tax for students? Can I clain a portion of it back?

Also if I leave the company within a year, I have to return the joining bonus. How does this work?

For ANY type of bonus, there is typically a 25% withholding for federal income taxes. The state withholding depends on the state.

You will claim the withholdings on your federal and state income tax return as reflected by the Form W-2 which you will get in January 2011.

Now, if you leave the company and have to return the joining bonus, you will either have to get the W-2 adjusted (with Form W-2C) and amend the 2010 tax return OR have the bonus debited from your 2011 salary.