Recent developments in modelling gas-phase catalyzed olefin fluidized bed polymerization reactors are critically examined in order to quantify the effect of the mean particle size on the dynamic behaviour of the FBR A comprehensive population balance model is developed to investigate the effect of particle size distribution in gasphase fluidized bed olefin polymerization reactors. To assess the impact of mean particle size on the dynamic behaviour of an FBR, a new bubble-growth model is proposed. Simulation results show that model predictions are strongly influenced by the choice of the mean particle diameter, since its values dramatically affects the main fluidization parameters (e.g., solids terminal velocity, maximum stable bubble size, minimum fluidization velocity and the heat and mass transfer coefficients) in the bed.