The cluster size distribution and motion behavior in a fast fluidized bed have been measured systematically by using two optical fiber probes and a computer data acquisition system. The measurement results showed that (1) although clusters behave in a transient nature, sometimes aggregating and sometimes dispersing, the local time-averaged cluster lengths remain almost constant radially in the core region of both the upper dilute section and the lower dense section, but the local time-averaged cluster lengths in lower dense section are larger than those in the upper dilute section. (2) Cluster motion velocities are in direct proportion to the superficial gas velocity. In the lower dense section the probabilities of upward motion and downward motion are basically equal; but in the upper dilute section, most of the clusters move upwards. However, near the wall region of both the upper and lower section, the probability of downward motion is bigger than that of upward motion. (3) The fast fluidized bed is a bed of fluidized clusters. There are two phases in fast fluidized bed : a dispersed phase and a cluster phase. In the dispersed phase the particles present essentially as individuals, while in the cluster phase the particles agglomerate with one another and are enmeshed in the dispersed phase. The motion behavior of clusters in the lower dense section and the upper dilute section are similar to the motion behavior of particles in the lower dense region and the upper freeboard region of a bubbling fluidized bed, respectively.