A three-dimensional (3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors of Geldart B particles. Five kinds of quartz sand particles (d(p) = 0.100, 0.139, 0.177, 0.250 and 0.375 mm and rho(p) = 2480 kg.m(-3)) were respectively investigated, with the total mass of the bed material kept as 10 kg. The superficial gas velocity in the riser ranges from 2.486 to 5.594 m.s(-1) and the solid mass flux alters from30 to 70 kg.(m(-2).s)(-1). Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing. The results found four typical cluster structures in the riser, i.e., the macro stripe-shaped cluster, saddle-shaped cluster, U-shaped cluster and the micro cluster. The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction, while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction. Additionally, clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region. All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.