The effects of particle size on the solid flow pattern in gas-solid bubbling fluidized beds were investigated numerically using two-fluid model based on the kinetic theory of granular flow. In this regard, the set of governing equations was solved using finite volume method in two-dimensional Cartesian coordinate system. Glass bead particles with mean sizes of 880 mu m, 500 mu m, and 351 mu m were fluidized by air flow at excess gas velocities of 0.2 m/s and 0.4 m/s. For particle diameters of 880 and 351 mu m, the predicted characteristic times for solid dispersion were 0.14 s and 0.15 s, respectively, while characteristic times for solid diffusivity were 1.68 ms and 0.75 ms in the same order. Consequently, at identical time-sampling interval, for coarser particles, longer simulation time is required to achieve accurate solid flow pattern. Through examination of wide range of time-sampling interval from 0.1 ms to 40 ms, an optimum value of 10 ms with minimum simulation time was obtained for coarser particles. The predicted solid flow patterns at a simulation time of 7 s were in good agreement with the experimental data for both particle sizes of 880 and 351 mu m. In addition, it was demonstrated that the effects of particle size on the solid flow pattern should be investigated alongside the variations in the excess gas velocity. In detail, predicted solid flow pattern underwent significant change with the excess gas velocity in the bed filled with a small particle size of 351 mu m, but for larger particles no considerable change was seen. Moreover, the predicted gas bubble diameter and velocity were in agreement with experimental data. (C) 2014 Elsevier B.V. All rights reserved.