A semi-analytical method has been proposed to analyse the mechanism of the solid state polycondensation (SSP) process. Computation results show that for the industrial SSP process for poly(ethylene terephthalate) (PET), the overall reaction rate for a single pellet can be appropriately simulated by a model controlled jointly by diffusion and reaction rates. From the core to the surface of a PET pellet, the SSP rate increases monotonically due to a gradual reduction of the concentration of such by-products as ethylene glycol and water. However, the SSP rate at any location within the pellet is observed between a maximum reaction rate when the concentration of by-products hypothetically approaches zero and a minimum reaction rate when the diffusion rate is assumed to be zero.