A model for predicting solid mixing in two dimensional bubbling fluidized beds is proposed. Assuming that diffusional mixing induced by bubble motion is superimposed on the convective or circulative mixing originating from bubble distribution, a diffusion equation is solved numerically with the values of two parameters, the solid circulation rate and the dispersion coefficient, properly estimated. The comparison between calculated and observed values of solid concentration reveals that this model is applicable to simulate solid mixing in bubbling fluidized beds.