The structure, dye degradation activity and stability of oxygen defective BaFeO 3Àx was investigated. The pure perovskite BaFeO 3Àx was synthesized at 700 C. With further increase in temperature, the oxygen loss occurred, and then the instability of BaFeO 3Àx was improved due to the change in crystal structure. BaFeO 3Àx can degrade methyl orange in the dark, and the degradation rate can be improved under visible light due to the optical property of BaFeO 3Àx . This degradation was caused by the transformation from Fe 4þ to Fe 3þ to some extent which was related with the structural transition of BaFeO 3Àx . So the reducibility of BaFeO 3Àx functions in dye degradation. The oxygen loss accelerated the structural transition and the photo-assisted degradation activity of BaFeO 3Àx . The instable Ba 2þ also improves the instability of BaFeO 3Àx .