Mixed electronic-oxide ionic conductivity and the cathodic property of BaCO03 doped with La was investigated in this study for a solid oxide fuel cell (SOFC) operating at intermediate temperature, The electrical conductivity of BaCO03 increases systematically with increasing amounts of La doped for Ba site. Cathodic overpotential at 1073 K decreased with increasing La content and attained a minimum atX=0.3-0.4 in Ba1-XLaXCoO3, Since the cathodic overpotential of Ba0.6La0.4CoO3 was small at decreased temperature, Ba0.6La0.4CoO3 is the optimum composition for a cathode of intermediate-temperature fuel cells among BaCoO3-based oxides, Mixed conductivity and surface activity for oxygen dissociation was investigated with O-18-O-16 exchange reaction. It was found that BaCoO3 doped with La exhibits a large oxide ion conductivity and surface activity. When La0.8Sr0.2Ga0.8Mg0.Co-115(0.05) (0.4 mm in thickness) was used as electrolyte, the power density of a cell using Ba0.6La0.4CoO3 for the cathode at 873 K attained a value of 120 mW/cm(2), which is slightly larger than that using a conventional SM0.5Sr0.5CoO3 oxide for the cathode.