The modeling and simulation of a KW-class molten carbonate fuel cell with direct internal reforming is presented. When the stack system was assembled with an additional preheating design, the transient responses show the stack start-up behavior with respect to preheating effect. Since the most influential property of temperature could affect the performance of the stack system, our analyses demonstrate some specific results: (1) inlet flows at the anode and cathode with the aid of the new preheater can improve the start-up of the stack, and (2) the prescribed preheater configu-ration is an inverse response system.