A three-dimensional computational fluid dynamics (CFD) model, using an Eulerian-Eulerian two-fluid model which incorporates the kinetic theory of granular flow, was developed to describe the gas solid two-phase flow in fluidized-bed polymerization reactors. Corresponding simulations were carried out in a commercial CFD code Fluent. The entire flow field in the reactors was calculated by the model. The predicted pressure drop data were in agreement with the classical calculated data. In addition, the model was used to describe the solid holdup distributions, the bubble behaviors, and the solid velocity vectors in the free and agitated fluidized-bed polymerization reactors, respectively. The effects of the addition of an agitator on the gas solid flow behaviors were preliminarily investigated via the model. The simulation results showed that the addition of an agitator can strengthen the fluidization efficiency and reduce the operation stability of the bed. However, the simulation results also showed that the total fluidization quality of the free fluidized bed was higher than that of the agitated fluidized bed at a superficial gas velocity of 0.5 m.s(-1).