The influence of mass transport inside microreactors containing catalyst layers on the wall was investigated using a mathematical model. In the model, the microreactor was divided into two parts, bulk flow area and catalyst layer, where reactions only occur in the catalyst layer. The concentration profile in the bulk flow area was expressed with a dimensionless equation using Damkohler number and the modified Peclet number. The influence of diffusion in the bulk flow area was judged with the concentration profile. The influence of diffusion in the catalyst layer was determined by well-accepted Thiele modulus and effectiveness factor. Finally, the whole relation between the diffusion in the bulk flow area and catalyst layer was graphically interpreted with the plot of Damkohler number and Thiele modulus against the intrinsic rate constant and catalyst layer thickness. The thickness of the catalyst layer was evaluated with the diagram.