The effect of gas velocity (0.56–1.15 m/s) on radial voidage distribution and cluster properties is determined by using an optical fiber probe in the freeboard of a FCC regenerator (0.48 m-I.D. × 3.4 m-high). The normalized standard deviation of pressure fluctuation in the freeboard region decreases with increasing gas velocity. Local voidage distribution in the radial direction is relatively flat in the bubbling flow regime. However, in the turbulent flow regime, voidage in the core region is higher than that in the wall region. Clusters move up and down simultaneously, and cluster velocity in the core region is higher than that in the wall region. Cluster length decreases with increasing gas velocity. As the gas velocity is increased, cluster frequency, fraction and voidage decrease in the transition from bubbling to turbulent fluidization regime, but increase in the turbulent flow regime. Variation of the normalized standard deviation of pressure fluctuation in the freeboard can be related with the cluster properties. Cluster length decreases with increasing the suspension density. The cross-sectional mean cluster length in the freeboard zone is correlated with the suspension density as well as the experimental variables.