The ellipsometry was applied to estimate the thicknesses of the adsorption layer of water on tile surfaces of single crystals of silicon, Si(001) and Si(111) at a constant temperature of 20 degreesC. Water molecules adsorb more strongly on Si(111) than on Si(001) in the monolayer regime, whereas the opposit is tile case for the bilayer regime. The water adsorbed on Si(111) in the submonolayer regime does not desorb even when the vapor pressure of water is lowered down to 0.1 Pa, resulting in a strong hysteresis behavior. In contrast, water desorbs readily from Si(001) when the water vapor pressure is lowered. The air initio molecular orbital (MO) calculations with cluster models of the surfaces Si(111) and Si(001) were carried out to elucidate the characteristics of the adsorption of water observed by the ellipsometry, The calculations were executed for the physical adsorption and dissociative adsorption of water on Si(lll) and Si(001) in the monolayer regime, and the physical adsorption of water on the dissociative-adsorbed sites in the bilayer regime. The results from the ab initio MO calculations were qualitatively consistent with the experimental ones.