Aberration-corrected scanning transmission electron microscopy was applied to investigate two different film systems. It is found that when ultrathin PbTiO_3 films grow on the NdGaO_3 substrate buffered with La_（0. 7）Sr_（0. 3）MnO_3 electrode,the tetragonality is improved compared with PbTiO_3 films grown on NdGaO_3 substrates directly. It is owing to the accumulation of oxygen vacancies at the heterointerface between La_（0. 7）Sr_（0. 3）MnO_3 and PbTiO_3,induced by charge transfer of Mn valence reduction as evidenced by XPS analysis.The results may facilitate the development of nanoscale ferroelectric materials.