The surface effect on the dielectric polarization and refractive indices of BaTiO₃ ultrathin films on SrTiO₃ single crystal substrates is investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of the BaTiO₃/SrTiO₃ film is reduced as its thickness decreases. However, an electronic polarization appears within the SrTiO₃ substrate in the neighborhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO₃ bulk, is induced by the polarization of the BaTiO₃ film. Furthermore, we find the refractive index either for BaTiO₃ films or for SrTiO₃ substrates to be strongly reduced for light polarized perpendicular to the surface.