Investigation on the mixing behaviors of spherical particles has been carried Out experimentally. A visual flat-bottom spout-fluid bed with a cross-section of 0.1 m x 0.03 m and height of 0.5 to was built. And the axial and radial concentrations of tracer particle, sharing the same characteristics as the bed material. were obtained by means of a box-counting method and digital image processing technique. In addition, the effects of two representative operating parameters (spouting gas velocity and fluidizing gas velocity) on the overall mixing were discussed by assessing the mixing index, respectively. Meanwhile, the mechanism of mixing was preliminarily discussed. It is found that particle mixing process. according to time, could be divided into three sequential stages: macro-mixing stage, micro-mixing stage and stable mixing stage. Increasing Spouting and fluidizing gas velocity can both promote the mixing. However, the axial mixing is more vigorous than the radial mixing with the increase of spouting gas velocity and vice versa when increasing fluidizing gas velocity. Further analyses illustrate that for a given condition there exists a gas velocity producing the best mixing, which has relation to the flow pattern. Below this velocity mixing improves and above this velocity mixing worsens as the gas velocity is increased. (C) 2008 Elsevier B.V. All rights reserved.