Paiman Amiri said:

I am using GC Shimadzu 15A and the baseline goes downward after a while even in high temperatures, Anyone knows why?

There are a couple of common causes, but the detector can make a big difference. It could be as simple as having heavy material slowly bleeding off your column at elevated temperature. You can get this happening with ECD detectors when there is an air leak upstream of the column, a small amount of air (oxygen) gets in causing an elevated baseline at the start, as the GC heats up, flow decreases (assuming you aren't running with a "EPC" type system), so the baseline actually drops.