Abstract: In order to investigate the effect of particle size difference and density difference on the mixing mechanism of binary granular materials within a rotating kiln, a kinematic model of particles was established using the discrete element method, and the transverse mixing process of granular materials in the rolling regime within the rotating kiln was simulated. The mixing index was defined by contact-number between particles, and the effects of particle volume ratio and density ratio on the mixing index were analyzed using Hong's theory:competition between percolation and condensation. The results show that the percolation mechanism improves with increasing volume ratio σ between particles, but the condensation mechanism improves with increasing density ratio η. Segregation will occur in the radial direction of the rotating kiln no matter percolation or condensation plays the leading role resulting in the decrease of mixing degree. Percolation and condensation can balance each other for some reasonable particle volume and density combinations, where the binary granular materials could get uniformly mixed. There is a power function relationship between particle volume ratio and density ratio when percolation and condensation balance each other.