The kinetics of solid-state-polymerization of waste PET was studied by modeling the diffusion kinetics of EG. For simplicity in modeling, the equation of diffusion was assumed to be independent of crystallization rate. Plain sheet and sphere models were evaluated to find suitable model representing the shape of waste PET particle. For each model, the diffusivities of EG at 17$0^{circ}C$, 19$0^{circ}C$ and 23$0^{circ}C$ were calculated. The activation energy of plain sheet type for the diffusivities of EG was 22.97 kcal/mol and that of sphere type was 26.17 kcal/mol. While the kinetics of each model at 17$0^{circ}C$ and 19$0^{circ}C$ showed no difference, sphere model showed a better agreement with experimental data at 23$0^{circ}C$, probably due to the increase in the diffusivity of EG with temperature. A deviation was observed between the derived equation and experimental data at 23$0^{circ}C$, probably due to the oversimplification of the derived equation by ignoring the steady-change of crystallinity during the polymerization.