Doped LaGaO3 exhibits high oxide ionic conductivity. Doping of Sr far the La site and Mg for the Ga site is the most effective method for enhancing the oxide ionic conductivity of LaGaO3. The oxide ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3 was higher than that of Sc-doped ZrO2 and slightly lower than that of Bi2O3 oxide. Furthermore, electronic or hole conduction was negligibly small in the oxygen partial pressure region from 1 to 10(-20) atm.