Based on particle velocity and size information measured by phase Doppler particle analyzer (PDPA), an algorithm to calculate particle cluster properties for dilute gas-solid flow was developed. The aggregate properties investigated in this study including voidage inside particle clusters, occurrence frequency, time fraction and particle cluster velocity show heterogeneous distributions in both axial and radial directions, although different properties differ from core to annulus region of the fluidized bed in their sensitivities to the variation of operating parameters. Experimental analysis shows that particle cluster properties are closely related to local time-averaged voidage and turbulent fluctuation of particles. (c) 2005 Elsevier B.V. All rights reserved.