The commercial utilization of fluidized beds is usually limited by particle agglomeration and subsequent defluidization. In this paper, a new mathematical model based on force balance is proposed to predict the defluidization behavior of particles in fluidized beds. In this model, the cohesive forces between particles are characterized using particle apparent viscosity and the separating force is mainly determined by the drag force. When the cohesion force was equal to the separating force at different fluidization condition, the minimum fluidization velocity and defluidization temperature were obtained by the model. Further, the fluidization behavior of copper particles including the minimum fluidization velocity and the temperature under which defluidization occurred was examined in a laboratory’s fluidized bed reactor. Compared with the experimental data, the results predicted by the model represent good agreement.