The large size stack of MCFC (Molten Carbonate Fuel Cell) becomes possible with the large electro-electrolyte plates and the stack of the unit cells. In this case, the estimations of the performances could be done with mathematical modelings. In this research, the mathematical modeling of fluid dynamic behaviors in the gas channels of MCFC and that of the butterfly-type cell has been studied. The effects of sizes of gas channels and utilization rates (Ua) have been studied. It could be shown that it is safe to assume that the gas concentration along the direction of gas channel height is constant. Current density distributions in the parallel flow cell and the butterfly-type cell were decreased along the gas flow direction. In the parallel flow cell, current density distributions ranged between 0.0932 and 0.0528 [A/cm2]. In the butterfly-type cell, it ranged a little bit lower than that in the parallel flow cell. In the parallel flow cell, conversion distributions in the anode gas channel were increased up to 0.3672. In the butterfly-type cell, they were increased up to 0.3870. In the anode gas channel, conversion distributions between the parallel flow cell and the butterfly-type cell were very similar. The conversions in the butterfly-type cell were a little bit higher. In the cathode gas channel, conversion distributions in the parallel flow cell were a little bit lower than that in the butterfly-type cell. Temperature distributions increased along the gas flow direction. But, after a specific position, it decreased. Distributions of temperature in the butterfly-type cell (from 923.7K. to 923.5K.) were a little bit lower than those of the parallel flow cell.