LaGaO3-based perovskite oxides doped with Sr and Mg exhibit high ionic conductivity over a wide range of oxygen partial pressure. In this study, the stability of LaGaO3-based oxide was investigated. The LaGaO3-based oxide was found to be very stable in reducing, oxidizing, and CO2 atmospheres. Solid oxide fuel cells (SOFCs) using LaGaO3-based perovskite-type oxide as the electrolyte were studied for use in intermediate-temperature SOFCs. The power-generation characteristics of cells were strongly affected by the electrodes. Both Ni and LnCoO(3)(Ln:rare earth) were suitable for use as anode and cathode, respectively. Rare-earth cations in the Ln site of the Go-based perovskite cathode also had a significant effect on the power-generation characteristics. In particular, a high power density could be attained in the temperature range 973-1273 K by using a doped SmCoO3 for the cathode. Among the examined alkaline earth cations, Sr-doped SmCoO, exhibits the smallest cathodic overpotential resulting in the highest power density. The electrical conductivity of SmCoO, increased with increasing Sr doped into the Sm site and attained a maximum at Sm0.5Sr0.5CoO1. The cathodic overpotential and internal resistance of the cell exhibited almost the opposite dependence on the amount of doped Sr. Consequently, the power density of the cell was a maximum when Sm0.5Sr0.5CoO3 was used as the cathode. For this cell, the maximum power density was as high as 0.58 W/cm(2) at 1073 K, even though a 0.5 mm thick electrolyte was used. This study revealed that a LaGaO3-based oxide for electrolyte and a SmCoO3-based oxide for the cathode are promising components for SOFCs operating at intermediate temperature.