Micromixing performance in reactor is one of the key factors for fast reaction systems, such as fine chemicals, pharmaceutical and crystallization. In this work, a three-dimensional CFD model was developed for analyzing flow characteristics and micromixing efficiency in microporous tube-in-tube microchannel reactor (MTMCR). PDF transport model based on Lagrangian Monte Carlo method and a modified Finite-rate/Eddy dissipation model were used to build micromixing model, respectively. The effects of Reynolds number and flow ratio on micromixing efficiency were studied using Villermaux-Dushman iodide-iodate reaction system. The segregation index Xs is used to quantify the micromixing performance. The results showed that Xs decreased with the increasing of Re and the decreasing of flow ratio R, and simulated values agreed well with published experimental data. Based on the modified Finiterate/Eddy dissipation model, the parallel competing reaction system was analyzed by taking into account mixing-controlled and kinetic-controlled factors, which explained how this model worked. Moreover, porous diameter and width of annular channel were optimized to improve the micromixing efficiency of MTMCR, and the optimal values were 20 mu m and 250 mu m, respectively. (C) 2017 Elsevier B.V. All rights reserved.