Electron-surfon interaction is used to calculate the mobility μ of electrons in the lowest subband in N-channel Si MOS-FET on the (111), (110) and (100) surfaces and at around room temperature. The surfons are the quanta of elastic waves in isotropic elastic continuum (which is to represent Si) occupying a half-space and having a stress-free plane boundary. In the approximations of high temperature surfon distribution and of elastic scattering, the result is (i) that the dependence of μ upon temperature and surface electron concentration can be fairly well reproduced but (ii) that the magnitude of μ has turned out to be about 10 times as high as the experimental values so far as bulk values are used for the deformation potential constants and for the effective masses of electrons. The correction due to the presence of SiO2 layer on Si is estimated to be small. It can be argued that correction due to intervalley scattering and electron population in the upper subbands are in the direction of improving the fit to the experiments. Still, the magnitude of μ remains unexplained unless the values of the deformation potential constants and effective masses of electrons differ in the surface region from the corresponding bulk values.