The purpose of this research is to elucidate quantitatively the degree of performance improvement in solid oxide fuel cells (SOFCs) which use methane as a fuel by introducing anode off-gas recycle (AGR) through experiments and simulation. In the experiments, power generation characteristics of a commercially available disc-type SOFC under AGR conditions were measured using an apparatus that can simulate an anode feed gas for AGR conditions with high accuracy. Because the experiments are conducted using the cells with seal-less structure, the possible diffusion of surrounding air into anode negatively affects the cell performance. This effect of air penetration which may appear in experimental data was evaluated by numerical simulation and taken into consideration. As a result of the experiment and the simulation, an improvement of the cell voltage by conducting AGR was evaluated and factors of the performance improvement were quantitatively discussed. (C) 2016 The Electrochemical Society. All rights reserved.