The dynamics of solids mixing in a fluidized bed is investigated. Both radial mixing and vertical mixing are taken into account. In the model solids, which consist of an upward moving wake phase (W) and a downflowing emulsion phase (E), can exchange between the two phases, and diffuse in the E phase. Also, properties of bubbles, solids diffusion coefficient and drift velocities are variable along with the height of the bed. The power-law difference method is used to solve the convection-diffusion equations. The numerical results are in satisfactory agreement with existing previous experimental data.