A systematic study magnetic properties of CoPt alloy thin films as a function of composition and substrate deposition temperature has been carried out. The concentration dependence of the intrinsic perpendicular magnetic anisotropy constant K-u at ambient temperature reveals a maximum at about 25 at% Pt. Films of Co75Pt25 deposited onto Al2O3(0001) substrates held at temperatures from 180 degrees C to 400 degrees C exhibit a strong perpendicular magnetic anisotropy (1.5x107 erg/cm(3)). This strong anisotropy is closely related to the ordered state of hcp structure.