The uniform mixing of solids is important in many industries,such as the pharmaceutical,food,petro-chemical and chemical industries.We numerically investigated the effect of particle size ratio on the mixing of bisized particles in a quasi-two-dimensional vibrationally fluidized bed.The granular binary mixtures comprised spherical particles with different size ratios.Three-dimensional discrete-element simulations agreed with previous experimental results.Convective and diffusive mechanisms occurred within the vibrated bed.The particle size had no significant influence on convective mixing,whereas the diffusive mechanism strengthened for large size ratios.The average particle velocity was larger in a mixture of large size ratios.The stronger diffusive motion and larger average particle velocity caused the particles to mix faster for large size ratios.The final mixing index decreased with size ratio because of the difference between the size and number of small and large particles.