Effects of doping certain transition metal cations for the Ga site on the oxide ion conductivity of La0.8Sr0.2Ga0.8Mg0.2O3 has been investigated in detail. It was found that doping with Co is effective for enhancing the oxide ion conductivity. In particular, a significant increase at low temperature was observed. Electrical conduction can generally be improved by doping transition metal cations, however, the theoretical emf was almost exhibited in an H-2-O-2 cell utilizing Co doped La0.8Sr0.2Ga0.8Mg0.2O3 for Co concentrations lower than 10 mol% on the Ga site. Diffusion characteristics of the oxide ion in La0.8Sr0.2Ga0.8Mg0.5Co0.085O3 has also been investigated by using the O-18 tracer method. Since the diffusion coefficient measured by the O-18 tracer method is almost the same as that the one estimated via electrical conductivity measurements, it was concluded that the conduction of La0.8Sr0.2Ga0.8Mg0.115Co0.085O3 is almost ionic. It seems most likely that the improved oxide ion conductivity by doping with Co is brought about by the enhanced mobility of the oxide ions.