Abstract: A CFD-DPM method has been employed to investigate the influence of initial particle height to bed width (i.e., bed aspect ratio) on the particles mixing trend in a gas-solid fluidized bed. Moreover, the effect of particle size and density on the particles mixing have been investigated. Particles density has no considerable effect on the mixing time, but mixing time increases by increasing the particle diameter. The simulation results show that the initial bed aspect ratio has no considerable influence on the lateral mixing index, but in the axial direction at reduced bed aspect ratios the particles mix very well. It was found that changing the bed regime toward the slug flow is the main reason for poor mixing indices at higher bed aspect ratios. A correlation is proposed for prediction of the mixing index in single bubble regime on the basis of some dimensionless groups.