To realize the commercial Molten Carbonate Fuel Cell (MCFC) plant, it is necessary to estimate the performance of MCFC during 40, 000 hours and improve the voltage decay rate to final target level (0.25%/1000 hours). We analyzed the factor of performance decay through operating bench-scale cells for 3, 000_??_10, 000 hours and obtained following results. The main factors of cell performance decay are identified to increase internal resistance and cathode reaction resistance. Suppose these resistances are expressed by Arrhenius type equation, the increase of resistances to operating time is caused by that of frequency factor in respective resistances, while the activation energy in respective resistances is appeared constant to operating time. The time dependence on increasing resistances is seemed to be linear (αt^1.0) since 3, 000 hours operation, and the increase rate of resistances would be rearranged by Arrhenius plot about operating temperature. As the above results, we propose a model for estimating MCFC performance with operating time.