A modified mathematical model based on the melting and recrystallization of an initial distribution of melting temperatures satisfactorily predicts the melting behavior of PET in differential scanning calorimetry. The simulated DSC curves produced in this work agreed fairly well not only with experimental DSC curves performed by Holdsworth et al., but also with the DSC traces provided by SSP PET pellets. The model, taking into account the initial distribution of melting points and the distribution of melting points for the recrystallized material, succeeded in analyzing the origin of dual endotherms of PET with various thermal histories, thereby elucidating the effect of changing crystallization temperature and time, the heating rate in the DSC heating scan, as well as the SSP process on the melting behavior of PET completely. Furthermore, it has been analytically proven that the crystallinity measured on a DSC diagram could not be equal to the weight percentage of crystalline state in the initial specimen. The deviation of the measured crystallinity, as observed relevant to the melting and recrystallization processes, is caused by the changes of the heat of fusion with the melting temperature as well as the difference of heat capacities of liquid and solid-state polymer.