The investigation on the cluster velocity and size was conducted in a 2-D circulating fluidized bed by using FCC, glassbeads, and sand particles under various operating conditions. Digital image system, 2-channel and 4-channel optical fiber probes were used to identify particle aggregates in the riser. A new method was developed to calculate the cluster size and cluster velocity by analyzing the video images and the optical fiber probe signals. The cluster sizes and velocities were also obtained for particles with different density, size, and sphericity.