Partial electronic and oxide ionic conduction in LaGaO3 doped with Sr and Mg, Co for Ga site was studied with the ion blocking method. It was found that doping small amount of Co into Ga site is effective for elevating the oxide ion conductivity. However, it is seen that the partial electronic conduction monotonically increases with increasing Co amount and P-O2, at p-n transition was shifted to lower value. Even at X = 0.1, the oxide ion conductivity in LSGMC is still dominant. Calculation on the theoretical leakage of electrolyte of solid oxide fuel cells suggests that the highest efficiency of the electrolyte was achieved around 100 mu m in thickness for La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC). Preparation of LSGMC film on Ni-Sm0.2Ce0.8O2 porous anode was studied with the colloidal spray method. In order to prevent the reaction between substrate and film, La doped CeO2 was used for the interlayer film. In accordance with the theoretical calculation, open circuit potential of the cell using LSGMC film electrolyte with 40 mu m thickness becomes much smaller than the theoretical value. However, fairly large maximum power density (0.21 W/cm(2)) can be achieved at 873 K and even at 773 K, the maximum power density of the cell as high as 0.12 W/cm(2) was exhibited on the SOFC using 40 mu m thickness LSGMC electrolyte. (c) 2006 Elsevier B.V. All rights reserved.