My employer pays me for 80 hours of work every two weeks , in the form
of a payroll check . Then any hours that i work overtime he pays me
cash ,but he does not pay me overtime , it is my regular , just cash.
So what do i do about this situation?

Tell him you don't like it & ask him to sort it out.

File a complaint with your state (or other jurisdiction if not in
U.S.) dept. of labor.

The IRS would also be interested.

First, let us know where you are, State, country.
It shouldn't be too difficult the find the applicable law.
Employment laws on overtime are pretty clearcut,  
Before you approach your employer, you want to know just what your
rights are, and to be able to tell him what his risks are.  Maybe he
already knows, but it is polite and less aggressive to suggest that
maybe he doesn't, allowing him to save face and allowing you to try to
keep the discussion from becoming too confrontational.
Since you probably don't have a verifiable record of your overtime
hours for the past, you may have to settle with improving the
situation for the future.  Even if you can prove unpaid overtime for
the past, the tax problems for you and your employer may be
unattractive, especially for him, although that is your ultimate
threat.  You'll know if it is worth making it.

Let us know where are.

Good luck.

95124 Ca?  You can file a wage claim with the DLSE. 
http://www.dir.ca.gov/dlse/DistrictOffices.htm
But
You should consult with a an employment lawyer(they can get ALL the
pay, penalties, and fines you are owed.
http://www.gotovertime.com/welcome.html

I wouldn't waste any time. It could cost you.

As Joe916, Myoarin and Nelson have said invarious ways, this could
cost you unless you have been meticulously reporting the cash payments
to the IRS and adding the required deductions to your tax payments.

If you figure out the tax and other liabilities I expect you'll find
you come out pretty close to even being paid straight hourly for your
overtime, plus you get to enjoy all the money right away.

I am not advocating cheating the government, mind you (and whatever
Big Brother is watching).