For better understanding of particle mixing behavior in fluidized beds, the concentration of tracer particle was measured in a rectangular fluidized bed to examine horizontal and axial particle mixing characteristics in a fluidized bed. The experiments were performed both by taking images at the bed wall with a CCD camera and by analyzing the images with an image digitizer. Two kinds of lime stone particles were selected to examine the influence of particle size on the mixing. As a result, a large scale particle circulation developed in the bed largely affects on solid mixing in bed wall region. The mixing in the center region and bottom region of fluidized bed are governed by movements of bubbles.