A new surface potential form is proposed to describe the action of the rubbed substrate to the liquid crystal molecules. By means of the molecular field theory, thin liquid crystal films confined by two rubbed substrates which lead to a homogeneous planar alignment of the liquid crystal molecules are studied. Adopting the Lebwohl-Lasher model,the molecular centers of mass are located at the sites of a simple cubic lattice. Numerical calculations are made for a liquid crystal film composed of 20 molecular layers. Three states with the different preferred directions of the molecular orientations exist and the one oriented along the rubbed direction has the lowest free energy. In addition, the symmetry of the biaxiality induced by the surface action is considered.