Large coercivity of 4.8 KOe with fine grains and smooth surface was obtained in Co0.27Pt0.73 alloy film deposited directly on heated glass by sputtering an alloy target at ultrahigh Ar pressure of 20 Pa. The effect of Ar pressure on the microscopic structure and magnetic anisotropy was investigated. The increase of the coercivity with the sputtering pressure was attributed to the increase of the perpendicular magnetic anisotropy but not the change of microscopic structure. The similarity of Kerr hysteresis loops of a film on and without substrate showed clearly that the perpendicular magnetic anisotropy did not result from external strain. Strong evidence is shown that there was Go-rich phase segregation in the films with strong perpendicular magnetic anisotropy.