An extension of the effective reaction zone of the Ni-zirconia cermet anode in a solid oxide fuel cell was investigated. The reaction zone, the active three-phase zone (ATZ), of the cermet electrode consists of the zone that is formed on thr flat interface between the solid electrolyte and the cermet layer, ATZ(2D), and another that extends three-dimensionally into the cermet layer, ATZ(3D), from the cermet/electrolyte interface. Anode performances of four types of cermet electrodes with zirconia powders of different ionic conductivities, but with a very similar microstructure, were compared, and the magnitude ratio of ATZ(3D), and ATZ(2D), was evaluated from the relationships between the performance and the ionic conductivity of zirconia in the cermet, For the anode of Ni-YSZ (yttria-stabilized zirconia 8 mol % Y2O3) cent on partially stabilized zirconia (3 mol % Y2O3) electrolyte, it was found that the contribution of ATZ(3D) was almost 2.5 times larger than that of ATZ(2D) at 1273 K, H-2-H2O 2.4%.