Mechanical properties are considered as critical factors for electrochemical applications of ion conducting oxides because chemically induced stresses can cause component degradation, and it also needs to reduce the thickness of conductors for higher performance. Mechanical properties and structural stability under a reducing environment of oxygen-permeable, dense membranes were investigated. The mechanical properties such as fracture strength and toughness of the membrane could be explained with microstructural features and relative density. Reasons for membrane failure occurring during an oxygen permeation experiment were discussed with viewpoints towards lattice parameter change caused by nonstoichiometry, phase instability under the reducing environment, and stresses induced by reactor configuration. Stress analysis and fractographical investigation were conducted to support the discussions. It was found that a Sr- and Fe-doped LaGaO3 membrane is reliable under a reducing environment at elevated temperature.