A method for designing membrane reactors with microchannels is discussed. The design equations were simplified under various assumptions, and a simple model that only incorporates the material balance equations was developed. The activity of the catalyst and the membrane capacity are important properties of a membrane reactor. In the proposed design method, these two parameters are combined into the nondimensional number ?. The simple model was applied to the water-gas shift reaction and the results were compared with those obtained from the detailed model by computational fluid dynamics calculations. Moreover, a reactor was designed that could achieve a CO concentration of 10?ppm or less. In this plate-type reactor, the temperature distribution in the channel can be controlled by adjusting the wall temperatures.