Cluster formation in the risers of circulating fluidized beds has been verified by experimental works of different investigators. In this work, a model has been developed based on the force and mass balances over the riser and core cross section in order to characterize the core-annulus structure of the fully developed zone by the cluster-based approach. The clusters are considered to be spherical with constant diameter. Radial profiles of cluster velocity, solid mass flux, and solids holdup have been obtained by this model as the intermediate variable. The comparison is made between the results of the model and the experimental data and modeling reported in the literature. The model is able to explain the experimental data satisfactorily, and the core radius which is the key parameter in the core-annulus flow structure has been estimated. The results of this model could be used to understand the flow structure in the riser of a circulating fluidized bed largely adapted for industrial applications.