How behavior of clusters in circulating fluidized beds was predicted by a proposed two-fluid model using a cluster-based approach (CBA). The model was assumed that the particles move as clusters in the riser. The cluster sizes were estimated from the existing correlations found in the literatures. The distributions of pressure drops, particles concentrations and both gas and solids velocities, and cluster diameters were predicted. A core-annular flow structure in risers was observed from simulations. The numerical results were in agreements with experimental data of [Manyele et al., 2002. Chemical Engineering Journal 88, 151-161] and [Knowlton 1995. Workshop at the Eighth Engineering Foundation Conference on Fluidization]. (c) 2005 Elsevier Ltd. All rights reserved.