The mixing operation that reactant fluids are split into many fluid segments to shorten diffusion length and thus mixing time is often used in microreactors. In this paper, we introduce two dimensionless numbers that represent effects of geometric design factors for the fluid segments, that is, arrangements and shapes of the fluid segment cross section at the reactor inlet, on the product yield for the reactors using this mixing operation. We also devise methods to determine the parameters. The dimensionless numbers are the ratio of reaction rate to diffusion rate φ and the aspect ratio of the mean diffusion length to the two-dimensional directions in the fluid segment w. To examine the validity of these parameters, we compare the yield of the desired product for a reactor having an arrangement and a shape of fluid segment cross sections at the reactor inlet with that for the reactor having rectangular cross section arranged in two-dimensional directions of the same dimensionless numbers using CFD (Computational Fluid Dynamics) simulations. The results show that the effect of arrangements for square cross section can be expressed by w, and that the order of φ allows us to determine whether a reactor is operated under the reaction-controlled condition even when the cross-sectional shapes are changed. Moreover, the effect of w on the product yield for cross-sectional shapes applied in this study, namely rectangle and right-angled triangle, is different when φ is fixed.