Abstract: Instantaneous images of gas - solid flow structure in High Density Circulating Fluidized Bed (HDCFB) are presented by means of a one -dimensional optical -fiber image analytic system specially developed for this experiment. The research is carried out in a circulating fluidized bed 9 m in height and 186 mm in ID. The operating conditions are in the range ug:0-8. 0 m/s, Gs:18-180 kg/(m2 · s) and bed voidage: 0. 65-0. 99. The distributions of the cluster characteristics along the radial direction,such as the probability of the cluster appearance, the radial cluster size, and the axial cluster interception time, are obtained through the statistics and correlation analysis of the cluter images. It is shown that the cluster characteristics are mainly affected by bed density and gas velocity. When gas velocity and solid flux are kept constant, radial cluster size, axial cluster interception time and the probability of cluster appearance decrease along the bed height, i. e. ,they decrease with decreasing bed density. When bed density is kept constant, all the above three cluster parameters decrease with increasing gas velocity.