In the modeling of the solid-state polycondensation, the degree of dependence on the reaction step and diffusion step depends on the size of the prepolymer pellet. In other words, the model of the solid-state polycondensation processes should be a function of the size of the prepolymer pellet. In the present work a hybrid model based on the fuzzy techniques is proposed to represent simultaneously the characteristics of the reaction step and the diffusion step of the PET SSP process. The fuzzy technique was employed to take account of the “medium” particle size. The simulation results show the effectiveness of the hybrid model proposed. For the small pellet size, the hybrid model represents the behavior of the reaction-controlled step while the hybrid model depicts the behavior of the diffusion-controlled step for a large pellet size.