We have investigated the growth of thin selenium layer on Si (100) substrate by molecular beam epitaxy (MBE). By controlling the temperatures of the silicon substrate and the selenium source during growth, an ultrathin film of Se is successfully grown on the Si (100) substrate. As the Si (100) surface is passivated by the ultrathin film of Se, the electrical property of the Ti/n-Si (100) contact is shown to be ideally ohmic, with low resistance and relatively high thermal stability.