The relationship between the incident angle of a laser beam with linear polarization and absorptivity on S40C steel at a wavelength of 10.6μm was investigated by both laser calorimetry and theoretical calculation based on the Fresnel equations. A simplified triangulate surface model and a highly polished specimen were used to determine the effect of surface roughness on absorptivity. A comparison was also made with a normally machined specimen to confirm the effect. With regard to the absorption characteristics, it was found that the calculated and the measured absorptivities for an ideally flat or highly polished surface were nearly the same over the entire range of incident angles examined. Absorptivity was strongly dependent on incident angle. A pronounced absorptivity peak was observed near the Brewster's angle which was approximately 86° from the normal. When the surface was asymmetrically triangulated, the angle at which peak absorptivity occurred was either larger or smaller depending on the incident direction of the laser beam. The peak absorptivity value on triangulated specimen was lower than that on the polished specimen. These tendencies agree closely with the predictions made by theoretical calculation and with the results for the normally machined specimen.