The molecular field theory is used to invesigate the behaviour of the nematic alignment transition within nematic liquid crystal (NLC) films confined by two rubbed substrates. The molecular centres of mass are located at the sites of a simple cubic lattice. The NLC consists of polar molecules and the molecules of the first layer of the NLC adjacent to the soild bounding surface are affected by both surface dispersion and polar forces. From self-consistent numerical calculations, three types of phase diagram are obtained, which exhibit a transition from a high temperature homogeneous alignment to a low temperature homeotropic alignment. The parameter ranges of two surface interaction which leads to the homogeneous-to-homeotropic alignment transition is obtained. The results show that the surface dispersion action, not the surface polar one, can be changed by the grating of the substrate and the surface polar interaction can give rise to the homogeneous-to-homeotropic alignment transition.