Requirements on the clean energy are increasing due to the global warming. Various studies on fuel cell systems have been carried out to satisfy the demand. In this study, we predicted the performance improvement of the molten carbonate fuel cell system based on the 2.5MW class commercial model. The commercial system includes a pre-converter, reformer, cell stacks, afterburner and some miscellaneous components. The enhanced system recirculated combustion gas and anode off gas to improve efficiency. The variation of the recirculation ratio was analyzed to obtain the optimum performance.