A thrice-dimensional numerical analysis model of cell voltage, temperature and current profile In a molten carbonate fuel cell (MCFC) stack has been developed. ＇The Formula for MCFC Performance＇ that was derived hom the test results of a single cell having the same active components as the stack has been applied to the calculation of electrochemical performance. Calculation result of cell voltages and temperature profile showed good agreement with 10-100 kW class stack experimental data. Using this model, cell voltages, temperature profile and net output powers (MCFC output power minus required cathode recycling power) of five gas flow geometries have been compared. The calculation results showed that the net output power is highest in the co-flow geometry. From the viewpoint of a stack performance, a cooling power, and a control of maximum temperature, the co-flow type stack has advantages over the external reforming type MCFC power plant.