Abstract: A reaction and diffusion model is developed for the process of solid-state polymerization(SSP) to increase the molecular mass of poly(L-lactic acid)(PLLA).Three reactions are considered: forward and backward polycondensation reactions and thermal degradation reaction.The rate constants of these reactions and diffusivity of water are obtained by fitting the model to experimental data,on the basis of which the activation energies were estimated.Degradation is found to be insignificant at low reaction temperature, while at high reaction temperature it becomes important and cannot be neglected.The diffusivity of water has an important effect on molecular mass growth.Small particle is favorable to obtain a high molecular mass product.