Measurements of the solid mass fluxes and particle size distributions were made by using a suction probe in a pilot-plant Multi-Solid Fluidized Bed (MSFB) combustor. The focus of this paper is to understand the flow structure in a MSFB under combustion conditions and the effect of geometrical configuration of the combustion chamber on the internal flow structure and particle size distributions, This paper also presents the applicability of suction probe technique for measuring the particle flow behavior in a hot model MSFB. The measurement results on the radial solid mass fluxes and particle size distribution show that not only the radial solid mass fluxes but also the mean diameter of particle and radial distribution of coarse particles (i.e. large coal particles (> 1 mm)) depend on the geometrical configuration of the combustion chamber, The suction probe technique is found to be successful in measuring solid mass fluxes and particle flow behavior under combustion conditions.