The amount of water molecules adsorbed on solid surfaces was determined in order to carry out precise mass measurement of mass standards under vacuum condition. The experimental data showed that the amount of the adsorption on a stainless steel surface was about 6 times larger than that on a single-crystal silicon surface treated with chemical mechanical polishing. This result can be interpreted as follows: the surface roughness and grain boundaries of the stainless steel surface influence the amount of adsorbed water.