A numerical simulation is performed in this study for a plate-type microreactor with parallel microchannels and diagonal inlets/outlets. The methanol steam reforming reaction is considered, and the performance is evaluated by examining the concentration profiles of methanol, hydrogen, And carbon monoxide within the rector under various operating conditions, including the influences of the steam/carbon (S/C) ratio, reaction temperature, and liquid feed rate. Particularly, the effect of aspect ratio of the microchannel on the methanol conversion rate, hydrogen generation rate, and mole fraction of carbon monoxide in the outlet gas mixture are also explored. The results show that such a plate-type design of methanol microreformer may cause nonuniform reaction states in each microchannel and affect its reforming performance significantly. It is found that these concentration nonuniformities are heavily dependent on the reaction temperature, S/C ratio, and liquid feed rate. The numerical model provides an efficient way to characterize the reforming reactions within the reactor, and the results will benefit the future design for a plate-type methanol microreformer.