The crystal fields lower the 3s3d0 electronic energy level of O-2 and make it hybrid with the 2pπ level at transition temperature to give a spontaneous polarization orbit according to the Jahn-Teller effect. In various polarized phases, the direction of each polarization orbit of three oxygens in unit cell was determined by the covalent bonds of BO6 octahedron. A correct ratio of the componeons of Ps in four phases of BaTiO3, and the screen constant which agrees with Slater's value for O-2 were given. For cubic to tetragonal transition, the theory was compared with Devonshire's theory.