Numerical analyses were conducted for competitive-consecutive reactions in laminar now to clarify the mechanism responsible for improvements to product yield with microreactors. The conservation equations of species were non-dimensionalized to derive the dimensionless numbers that governed the flow and reaction in microchannels. Three dimensionless numbers were derived: Peclet number Pe, i.e., the ratio of the diffusion time to the convection time, mixing-reaction number phi, i.e., the ratio of the diffusion time to the reaction time, and K, i.e., the ratio of the secondary reaction rate constant to the primary reaction rate constant. As a result of the numerical analyses of the competitive-consecutive reactions for multilayer laminar now in microchannels, under conditions where convection was more dominant than diffusion Pe > 10, the product yield of the primary reaction depended on phi and K. A diagram to evaluate product yield was prepared using phi and K. The product yield increased with decreasing phi and K.