A series of doped LaGaO3 perovskite type oxides is considered as the electrolyte of solid oxide fuel cells (SOFCs), due to their high oxide ionic conductivity. However, detailed defect chemistry, particularly the relationship between oxygen nonstoichiometry and lattice parameter under operating condition of SOFCs has not been understood well so far. To clarify the relationship, measurement was made on the oxygen nonstoichiometry of La 0.8Sr0.2Ga0.8Mg0.15Co 0.05O3-δ by high temperature gravimetry, and on the crystal structure by in-situ high temperature XRD. Oxygen vacancy concentration increases with decrease in log (P(O2)) and increase in temperature, finally converging on 3-δ=2.80. The large variation in the oxygen nonstoichiometry results from valence change of Co. The valence of Co at 3-δ=2.80 is nearly +2.0, whereas, the valence of Co at 3-δ=2.85 is considered close to +4.0. In-situ high temperature XRD confirmed that the oxygen vacancy formation led to increase in the lattice parameter, namely chemical expansion induced by oxygen nonstoichiometry.