In this work, we focused on La 0.95 Ba 0.05 Ga 0.8 Mg 0.2 O 3−δ with the perovskite structure, and investigated the local structure around the oxygen vacancy by pair distribution function (PDF) method and density functional theory (DFT) calculation. By comparing the G(r) simulated based on the DFT calculation and the experimentally-observed G(r), it was suggested that the oxygen vacancy was trapped by Ba 2+ at the La 3+ site at least at room temperature. Such a defect association may be one of the reasons why the La 0.95 Ba 0.05 Ga 0.8 Mg 0.2 O 3-δ showed lower oxide-ion conductivity than (La,Sr)(Ga,Mg)O 3−δ which was widely-used as an electrolyte of the solid oxide fuel cell.