The present paper discusses effects the fluid segments size on temperature profile 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 by the form of fluid segment. 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 cooing rate. The fluid segment height is then changed on the half way of reactor channel. Reactors where the height is small near the inlet region and enlarged suddenly after the region provides higher yield of the desired product than those where the height is fixed.