In order to investigate the surface SHG, (110) pure STO single crystals were exposed to a reducing atmosphere to induce the reduction of the Ti ion and the release of oxygen from the lattice compensating the reduction of the Ti ions. The anisotropy and asymmetry of SHG intensity explains a slight shrinkage. The incoming fundamental wave was polarized either in the $p$-in or $s$-in to the plane of incidence for the reflection geometry. The SH polarization diagram could be described by the electric dipole and/or quadrupole contribution of reduced STO single crystal surface.