We have investigated the changes caused by composition deviation at growing surfaces during epitaxial growth of BaTiO3 in terms of morphology and oxidation state of cations. Surface morphology was significantly changed by the Ba/Ti ratio. Ba-and Ti-rich surfaces were roughened by three-dimensionally grown islands. An atomically flat surface and well-oxidized cations were obtained at the stoichiometric surface. Surface lattice parameters of the stoichiometric (Ba/Ti 1) sample during epitaxial growth of BaTiO3 on SrTiO3 substrate have been investigated by in situ reflection high-energy electron diffraction (RHEED) observation. We found that the oxygen partial pressure significantly influenced the surface lattice strain relaxation and RHEED oscillation.