The oxygen-nonstoichiometry of La_1-xSr_xCoO_3-δ was investigated by X-ray photoelectron spectroscopy (XPS) analysis, thermal analysis (TGA-DTA) and transmission electron microscopy (TEM). The results of thermal analysis showed that the sharply weight losses in LaCoO_3-δ were decreased by Sr substitution. The Co 2p and valence band XPS spectra of LaCoO_3-δ suggested that the annealing at 523 K in 20% O₂-80% N₂ gas increased the oxygen contents and then low-spin ions. On the other hand, the Co 2p and valence band XPS spectra of La_0.8Sr_0.2CoO_3-δ were not changed by this annealing conditioned. Also, TEM observation indicated the change of crystal structure by Sr substitution. These results suggested that the Sr substitution affected the oxygen defects on the oxide surface.