A mixing model in a microreactor was developed by using the principle of lamination segments. For four types of multiple reactions, the effects of lamination width and rate constants on the relation between the conversion of reactant and the yield of desired product were investigated by using CFD simulations. The simulation reveals that the lamination width greatly affects the yield for the desired product. In most cases, when the conversion of the reactant is the same, the yield of the desired product for perfect mixing is higher when reactants are fed with lamination segments. An exception is that the rate constant of the reaction producing the desired product is much smaller than that consuming the desired product, and the order of the former reaction is less than the latter. In this case, the feed with large lamination width shows higher yield of the desired product than the feed with perfectly mixed. In this study, the maximum lamination width which shows almost the same performance as the case of perfect mixing was derived for various multiple reactions and reaction conditions. The results provide effective information for the design of microreactors and of the measurement system of rate constants of multiple reactions. (C) 2004 Elsevier B.V. All rights reserved.