A series of FePt thin films with perpendicular c-axis orientation were deposited on single crystal MgO (001) substrates by DC magnetron sputtering. With the increase of substrate temperature during deposition, FCC disordered structure transforms to FCT ordered structure (L10 phase) gradually, and the chemical ordering parameter increases simultaneously. Furthermore, the magneto-crystalline anisotropy, coercivity and squareness ratio of the hysteresis loops along perpendicular direction increase also. The magneto-optical Kerr spectra were found to be changing notablely with the chemical ordering of FePt films.