Abstract: The optical constant of alloy steel was measured with ellipsometry when the incidence wavelength was 0.632μm and incidence angle was 50°～80°. Since the surface of the alloy steel was rough, the measured optical constant was corrected with the Ohlidal-Lukes theory. It is founded that the correction of the ellipsometric parameter increases proportional to the incidence angle. The results indicate that the calculated result after correction agrees with the measured one when the incidence angle is in the range of 50°～70°, but differs significantly when the incidence angle is in the range of 70°～85°. Therefore, the accurate optical constant of alloy steel can be obtained by ellipsometry when the incidence angle is less than 70°.