This paper reports the results of a computational fluid dynamics (CFD) simulation for a sequential heterogeneous catalytic reaction by comparing the reaction selectivity in a catalytic microreactor with that in a conventional catalytic packed-bed reactor. The comparative simulation revealed that the catalytic microreactor has an advantage over the conventional reactor in the diffusion control regime, and that the control of concentration distribution of reactants by molecular diffusion in the microspace can enhance the selectivity of the desired product. The concentration distribution was intentionally controlled by the shape of the microreactor. The yield of the desired product in the microreactor was 1.16 times that in the packed-bed reactor. CFD simulation of methanol decomposition was also performed by fitting the parameters with the experimental results. The results of the CFD simulation verified the advantage of the microreactor.