Micrographs in a 10-m high and 90-mm ID fluidized catalytic cracking (FCC) catalyst-air fast fluidized bed were taken by a video camera provided with an optical fiber probe inserted radially into the bed to study cluster size and shape in gas-solid suspension. A computer aided (CA) particle image processing system was used to analyze images which were obtained under different superficial gas velocities and solid circulating rates at different radial positions. Cluster size and its probability density distribution were investigated and a simple model was established. A roundness factor was put forward to describe the shape of the cluster. We also discovered that clusters assembled by particles show evident fractal character both in ’shape’ and ’fragmentation’. The fractal dimension of clusters at different flow patterns was found to be a function of position.