The planar solid oxide fuel cell (SOFC) with a Y2O3-stabilized ZrO2 electrolyte is expected to be a candidate for distributed power sources in the next generation due to its high efficiency of power generation. In this study, we analyzed the system performance of a SOFC/gas turbine combined cycle of about 500 kW electrical output, using our two-dimensional simulation code for the planar SOFC with internal reformer. The effects of cell temperature, cell pressure, recirculation rations of fuel and air, utilization ratios of fuel and air, and average current density of SOFC on both the system efficiency and the cell temperature and current density distributions, were calculated under typical operating conditions taking account of realistic efficiencies and heat losses for auxiliary equipment. The addition of a Cheng cycle to the SOFC/gas turbine combined cycle improved system efficiency by 1-3%. The combined SOFC/gas turbine/ Cheng cycle gave a high efficiency of 61.2% (based on a higher heating value! even under a small power generation scale of 500 kW class at 2.0 MPa SOFC pressure. We also discussed the possibility of carbon deposition at both external and internal reformers by calculating the chemical equilibrium carbon activities for estimated carbon deposition reactions. (C) 2003 The Electrochemical Society.