Particles-liquid flow involving a free surface in partially filled rotating cylindrical tank is common in chemical engineering, which attracts numerical studies on it in an effort to provide useful information for the design and optimization of operational conditions in the real application. However, the dynamics of particles dispersion in partially filled rotating cylindrical tank under small particles-liquid density ratio has not been fully understood by performing a reasonable 3D numerical simulation. A three-dimensional unsteady model of a solid-liquid dispersed flow involving a free surface has been developed and numerically analyzed to investigate the particle dispersion in a partially filled rotating cylindrical tank. In the model, the discrete element method (DEM) and the volume of fluid (VOF) method have been applied. The flow field and the particle dispersion process in a rotational cylindrical tank have been presented. The effects of the tank rotation speed and the liquid height on the dispersion behaviors of particles are examined and analyzed. The results indicate that vortexes have formed in the liquid phase that entrain the particles and induce their gradual dispersion throughout the entire region of the liquid phase. The average mass concentration of particles is high in the rotating-out side of the liquid phase and in the liquid-wall contact region, while it is low in the rotating-in side of the liquid phase and in the vortex generation region. In addition, as the rotation speed increases, the entrainment capability of the liquid phase has been enhanced, which improves the dispersion performance of the particles. The selection of an appropriate liquid height is beneficial for the enhancement of particle dispersion in a partially filled rotating cylindrical tank. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.