Recently I got hired as an contractor and my recruiting firm is charhing me 30% tax , my previous emplyer used to charge me only 13 % , I am on OPT can any one tell why they are charging 30%

QUOTE," i got hired as an contractor and my recruiting firm is charhing me 30% tax , . . ."------Your employer who contract you, a F1 visa holder, as independent contractor rather than an employee will need to know the your tax status in order to properly tax and report the income. It will be your responsibility to inform your employer with whom you have contracted what your tax status is. In general, a Non Resident Alien,I gues you are, is subjected to US Tax withholding of 30% on his/her US Source Income. So, If you, as a non-resident alien, receive income other than wages, then you should file Form W-8BEN with the withholding agent toclaim a reduced rate of, or exception from, withholding under an Income Tax Treaty.
However, if the income is effectively connected with a U.S. trade or business, you need to file Form W-8ECI instead.
In principle, you are not supposed to work as an independent contractor as a F-1 visa holder, but, as a substantial matter, it doesn't make any difference tax-wise, so that prohibition is usually disregarded.
However,it, fundamentally, depends on the revenue andexpense amount that you earn and spend. If the amount is relatively small, then it would probably not be a problem.