Abstract: Thickness effects of thin La_0.7Sr_0.3MnO₃ (LSMO) films on (LaAlO₃)_0:3(Sr₂AlTaO₆)_0:7 sub-strates were examined by a slow positron beam technique. Doppler-broadening line shape parameter S was measured as a function of thickness and differnt annealing conditions. Re-sults reveal there could be more than one mechanism to induce vacancy-like defects. It was found that strain-induced defects mainly influence the S value of the in situ oxygen- ambience annealing LSMO thin films and the strain could vanish still faster along with the increase of thickness, and the oxygen-deficient induced defects mainly affect the S value of post-annealing LSMO films.