Detailed knowledge of particle mixing dynamic is essential in successfully designing and reasonably operating spout-fluid reactor. Over the years, numerous methods for the investigation of particle mixing have been proposed. This paper presents a novel method for the investigation of particle mixing. The method was carried out in a well-designed microwave heater, in which the lab-scale spout-fluid vessel constructed with microwave-transparent Plexiglass was placed. Inside the bed, a number of polar tracer particles continuously heated by microwave energy were real-time tracked using a top-grade infrared camera. From this, the concentration field of the tracer particle could be obtained with high accuracy. This technique has been adopted to investigate the mixing dynamic of binary mixture differing in the density in a flat-bottom spout-fluid bed. The mixing quality was evaluated by the Shannon entropy mixing index. On this basis, the influence of important process parameters on the mixing progress in the initial mixing stage has been taken into account. The investigation results show that the mixing progress is improved when increasing the means gas velocity and the difference in density of bed particle. In addition, an empirical correlation as a function of gas velocity and particle density was developed for calculating the mixing degree. The errors between experimental data and the obtained ones from this correlation were within 28%. (C) 2011 Elsevier B.V. All rights reserved.