To estimate the size of agglomerates formed in fluidized beds of cohesive particles, a model of force balance is developed. The average agglomerate size estimated by this model is in reasonable agreement with experimental data. An agglomeration criterion is proposed by analyzing the model. Theoretical analysis shows that higher superficial gas velocity and fluid density, lower particle cohesion, and the collision between agglomerates are advantageous to the agglomerate fluidization of cohesive particles.