The present paper discusses the effects of the fluid segment size on reaction temperature as well as mixing performance in microreactors under nonisothermal conditions. In the reactors, exothermic parallel-series reaction systems having different ratio of activation energy proceeds with the cooling through walls of a fixed temperature. Reactant fluids are fed in the form of fluid segments. The width of fluid segment determines the diffusive mixing rate, and the height of fluid segment affects the cooling rate of reactant fluids. When the activation energy of the reaction producing the desired product is higher than that of the by-product, an optimum height to maximize the yield of the desired product exists with a fixed fluid segment width. This result indicates that the fluid segment size can be determined from the mixing rate and cooling rate. From the results on the reactor where reactants are mixed from the reactor inlet, even in a channel the scale of which is in the sub-millimeter range, suppressing hot spot is difficult when mixing and reaction are rapid and heats of reactions are large. For such reaction systems, mixing based on fluid segments is effective for controlling mixing and reaction rate. (C) 2008 Elsevier Ltd. All rights reserved.