A population balance model is developed to investigate the particle size distribution (PSD) developments in a gas-phase fluidized bed ethylene polymerization reactor. The model considers the combined effects of particle growth and elutriation for size-distributed prepolymer feed. In the proposed model, the bed is divided into several perfectly mixed serial sections. The population balance differential equations written for each section were simultaneously solved to determine the density function of the size distrubution of the polymer particles in each section. The model is able to predict the profiles of the PSI) along the reactor height. It was shown that the mean size of the particles is larger at the bottom of the bed and becomes smaller when moving toward the top of the reactor. The size distribution of the polymer particles in the product becomes sharper and their mean size decreases by increasing the superficial gas velocity. Also, the mean size of the particles in the product increases by increasing the temperature of the reactor. (C) Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008.