This work has been carried out to study the mixing characteristics of solid in vibrating feeder for stable operations of fluidized bed combustion. The system consisted of two particles such that fine particles were located on the top of the coarse particles before vibratory mixing had started. Effect of particle size, particle densities, vibration amplitude and vibration frequency were experimentally obtained. Also, a diffusion model was applied in interpreting the experimental results. From these results, the following empirical equation for the diffusivity was obtained. $0.87{(frac{d_c}{d_f})}^{0.73};{(frac{ ho_f}{ ho_c})}^{0.53}(A^2f)$