Oxygen vacancies introduced by homoepitaxial growth of thin films on SrTiO(3) substrates were studied by a monoenergetic positron beam. The SrTiO(3) films were grown by MBE without using an oxidant. The S parameter, corresponding to the annihilation of positrons in the substrate, was increased by the growth of the film. This fact was attributed to the diffusion of oxygen from the substrate into the film, and the resultant introduction of oxygen vacancies in the substrate. Isochronal annealing experiments were also performed to study the recombination between oxygen vacancies and oxygen atoms supplied from atmosphere.