Catalytic combustors are used as off-gas combustors of molten carbonate fuel cells (MCFCs) because of their exhaust gas purity, geometric flexibility, and high combustion efficiency. In this study, a new design was investigated for possible application in internally reformed MCFC. The study started with performance analysis of a 5 kWe combustor, which could be precisely conducted due to availability of experimental apparatus. A 5 kWe combustor was used as a model combustor, and it was experimentally analyzed in terms of flow uniformity, catalyst screening, and reaction characteristics. The results show that the flow uniformity is able to reduce the exhaust gas concentration because temperature uniformity decreases the possibility of fuel slippages in locally lower temperature zones. As the capacity of the combustor is increased from 5 kWe to 25 kWe, the exhaust gas temperature at the same inlet condition as that of the 5 kWe combustor increases due to lower heat loss. As a result, the catalyst screening process shows different results due to higher operating temperatures, but three of four catalysts provide proper quality. On the other hand, flow uniformity improves economic competitiveness of the catalytic combustor. When the volume loading of catalytic monoliths was decreased, the performance was very similar to that of the original volume loading of catalytic monoliths.