Power generation experiments were carried out to investigate the effective anode thickness and to clarify the mechanism determining the threshold current density for CO2 production. The production of CO2 increased fuel efficiency and reduced anode overpotential. The effect of anode thickness, diameter of NiO powder for anode, concentration of fuel and operating temperature on anode potential and outlet gas composition were measured in this report. The anode overpotential decreased with increasing anode thickness up to 120 mu m for pure hydrogen fuel and 70 mu m for 4.6% of methane fuel. The result indicated that the effective three phase boundary could be distributed from the electrolyte surface into the anode to a depth of 120 mu m for hydrogen fuel and 70 mu m for methane fuel. The threshold current density was explained by the surface activity with fuel on the three phase boundary. The modelling of the anode reaction by a complete mixing reactor consistently interpreted all experimental results.