Abstract: There is an interference effect between reaction and crystallization in solid state polycondensation(SSP) of poly(ethylene terephthalate)(PET).The crystallization behavior of PET with SSP was investigated using self-made oil-both reactor and differential scanning calorimetry(DSC).The crystallization temperature of the peak for PET pellets is higher and the enthalpy is smaller as the DSC heating rate increases.The results indicate that there is the melting-recrystallization-remelting process for PET at low DSC heating rate.The imperfect crystallites formed at lower crystal temperature.The crystallization structure has been observed by Nikon E600 polarizing microscope(with heating and freezing stage).The fractional crystallinity increases as the increase of the SSP temperature and reaction extent.There is a new crystal formation with the development of solid state polycondensation for PET pellets.