We investigated the atomic structures and ferroelectrics of ultrathin film of PbTiO3, whose ferroelectric polarization is parallel to the surface, through first-principles pseudopotential total-energy calculations. We examined the atomic displacements and found that, on TiO2-terminated surface, Ti, O-n, and O-p atoms are displaced dominantly along the direction normal to the surface, while on PbO-terminated surface, Pb and O atoms are displaced dominantly along the polarization direction parallel to the surface. Around the TiO2-terminated surface, the Pb-O hybrid bonding (one of the characteristics of ferroelectric lattice distortion) is weakened, while on the PbO-terminated surface it is enhanced. It is remarkable that the ferroelectric polarization is reduced around TiO2-terminated surface and, rather, enhanced on PbO-terminated surface. The surface effect is significant only within the layers of one lattice constant.