A novel solid oxide fuel cell (SOFC) utilizing partial oxidation of methane (CH4 + 1/2O(2) = CO + 2H(2)) as internal reforming reaction was investigated in the present study. Large electric power as well as CO-H-2 mixture with molar ratio of 2 were obtained by applying LaGaO3 perovskite as electrolyte. Although the open circuit potential decreased, the maximum power density increased by doping a small amount of Co to the LaGaO3 electrolyte. In particular, the increase in the power density at 1073 K was significant. Increasing the amount of doped Co monotonically enhanced the hole conduction resulting in a decrease in the open circuit potential and an increase on the amount of leaked oxygen which results in diminished electric power. Consequently, it became clear that the optimized composition for this electrolyte was La0.9Sr0.1Ga0.8Mg0.115Co0.085O3 considering the power density and the amount of oxygen leakage. Although the thickness of electrolyte with the above composition was as thick as 0.5 mm, a maximum power density and yield of synthesis gas were obtained at 242 mW/cm(2) and 16%, respectively at 1073 K.