AC impedance measurements were carried out with Ni-GDC cermet anode in various partial pressures of hydrogen at temperatures from 873 K to 1273 K; the data was analyzed focusing on the chemical capacitance. It is proposed that the effective reaction zone (ERZ) of the cermet anode could be evaluated by comparing the observed capacitance and the one expected for the whole electrode. The ERZ was estimated as 1 μm for the Ni-GDC electrode investigated in this work. Degradation mode of the cermet anode is discussed based on the observed change in the ERZ. It is suggested that the agglomeration of Ni particles might be the main factor for the degradation during the thermal cycle at high temperature.