Although hole conduction was present, it was found that doping with Ni was effective in improving the oxide ion conductivity in La0.8Sr0.2Ga0.8Mg0.2O3 based perovskite oxides. Considering the ionic transport number and the electrical conductivity, the optimized composition for Ni doped samples was La0.8Sr0.2Ga0.8Mg0.13Ni0.07O3 (LSGMN). In this composition, electrical conductivity was found to be virtually independent of the oxygen partial pressure from 1 to 10(-21) atm. Consequently, the oxide ion conductivity was still dominant in this optimized composition. In agreement with the improved oxide ionic conductivity, the power density of the solid oxide fuel cell using LaGaO3 as an electrolyte increased by doping with a small amount of Ni on the Ga site. In particular, the power density of 224 mW/cm(2) at 873 K, which is the maximum power density in the cells using LaGaO3 based oxide as the electrolyte, was attained using LSGMN in spite of the use of electrolyte plates with a thickness of 0.5 mm. Therefore, LSGMN is highly attractive for the electrolyte material of low temperature operating SOFCs.