Effect of fluidized bed geometry on solids mixing was investigated. Experiments were carried out in cylindrical and square fluidized beds. White and black particles with 850 mu m average diameter were used in the experiments in which black particles were considered as tracer. In order to determine the concentration of tracers in each sample, the method of image processing was applied. Based on the final value of the mixing index, it was concluded that complete random mixing can be achieved in both beds. The results indicated that mixing is faster and mixing time is shorter in the cylindrical bed than in the square bed. This trend was attributed to the fact that the movement of solids in the cylindrical bed is faster compared to the squared bed due to the presence of dead zones in the square bed which reduce the cross-section area available for passage of bubbles. A one-dimensional dispersion model was used to predict the solids axial dispersion coefficient. It was shown that the axial dispersion coefficient in the case of cylindrical bed is greater than that in the square bed and that the axial dispersion coefficient increases by increasing the superficial gas velocity in both beds.