Molten carbonate fuel cells (MCFCs) use molten carbonate as an electrolyte. MCFCs operate at high temperatures and have the advantage of using methane as a fuel because they can use nickel-based catalysts. We analyzed the performance of an internal manifold-type MCFC, according to operating conditions, using computational fluid dynamics. Different conditions were used for the external and internal reforming-type MCFCs. Flow directions, gas utilization, and operating temperatures were used as the conditions for the external reforming-type MCFCs. The S/C ratio and reforming area were used as the conditions for internal reforming-type MCFCs. A simulation model was developed, considering gas transfer, reforming reaction, and heat transfer. The simulation results of external reforming-type MCFCs showed similar pressure drops in all flow directions. As the gas utilization decreased, the temperature decreased, but the performance increased. The performance improved with increasing operating temperatures. The simulation results for the internal reforming-type MCFCs showed that more hydrogen was produced as the S/C ratio decreased, and the performance increased accordingly. More hydrogen was produced as the reforming area increased. However, similar performance was obtained when the reforming area contained the same active area. The external and internal reforming-type MCFCs were compared under the same conditions. The efficiency of the external reforming-type MCFCs is higher than that of the internal reforming-type MCFCs.