Doping transition metal cation is known to enhance the electric conduction of solid electrolytes, however, the ionic conduction can be improved by doping the small amount of transition metal, in particular, doping Co is effective for improving the oxide ion conductivity. In this investigation, oxide ion conductivity of LaGaO3 based oxide doped with Co were investigated in detail. It was found that LaGaO3 doped with Co for Ga site (LSGMC) show a notable oxide ion conductivity over a wide range of oxygen partial pressures, although a hole conduction was appeared by addition of excess amount of Co. Considering the electrical conductivity and transport number of oxide ion, the optimized composition of LSGMC seems to be existed at La0.8.Sr0.2, Ga0.8,.Mg0.115 CO0.085O3. Power generation characteristics of fuel cells was greatly improved by using LSGMC for electrolyte and extremely large power density can be obtained on both H2- O2 and H2-air cells. In particular, the maximum power density was attained to a value of 1.53 and 0.50 W/cm2 at 1073 and 873 K, respectively, on H2–O2 cell when the thickness of electrolyte was 0.18 mm. Furthermore, almost similar large power density was attained when air was used as oxidant. The high power density of cell demonstrated in this study suggests that the operating temperature of SOFC can be decreased by using LSGMC for electrolyte.