A catalytic combustor is a device to burn off fuel by surface combustion that is used for the combustion of anode off-gas of molten carbonate fuel cells by employing the catalytic combustor. Purified exhaust gas can be recirculated into the cathode channel for CO2 supply to improve thermal efficiency. The design of a catalytic combustor depends on many parameters, but flow uniformity is particularly important during the emergency shut-down of a fuel cell stack. Before the temperature control of a catalytic combustor is activated, the catalytic combustor should burn off more than two times the rated amount of the fuel flow rate. Under overload conditions, assurance of flow uniformity at the inlet of the catalytic combustor can reduce damage to the catalytic burner that can be caused by a local hot zone. In this study, flow uniformity of the catalytic combustor was investigated in two steps: a preliminary step with a model combustor and a main analysis step with a practical 250 kW catalytic combustor. Models of the 0.5 and 5 kW class combustors were used in the preliminary step. In the preliminary step the model combustors were used to determine supporting matters for flow uniformity. The inlet direction of the mixing chamber below the catalytic combustor was also examined in the preliminary step. In the main analysis step the flow uniformity of the scale-up combustor was examined with selected supporting matter and inlet direction into the mixing chamber. Geometric and operating parameters were investigated. In particular, the flow rate under off-design operating conditions was examined.