The surface effect on the electrical and optical properties of tetragonal barium titanate at room temperature is investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. Both the first-, second-, and third-order electronic polarizabilities were considered in this calculation in order to obtain accurate results for both the spontaneous polarization and refractive indices. All types of surfaces were studied, either being a- or c ^±-surfaces (both BaO- and TiO ₂-terminated). The spontaneous polarization is drastically reduced near the c-surfaces while it remains almost unchanged near the a-surfaces. Furthermore, both the a- and c ^±-surfaces show the tendency to deeply reduce the refractive index for light polarized perpendicular to the surface.