Porous La0.81Sr0.09MnO3 and LSM-YSZ composite electrodes of the solid oxide fuel cell (SOFC) were made, and the relationship between the overpotential and the three phase boundary (TPB) length was investigated by constructing a model. Three-dimensional distribution of TPB for the composite cathode was suggested. The effective thickness of it was observed to be less than 20 mu m.