Three sets of PET samples, comprising original (undrawn), uniaxially drawn and biaxially ones, after annealed at 230°, 240° and 250℃ respectively, were subjected to DSC thermal analysis, X-ray diffraction analysis and IR analysis. The results indicate that the phenomenon of double melting peaks during DSC analysis is due to the partial melting and recrystallization of the crystallite at the moment of thermal scanning. The lower temperature peak, which varies slightly according to annealing condition, corresponds to the melting of imperfect crystallite,and the higher temperature peak corresponds to the melting of better organized crystallite.In the course of temperature scanning,the unit cell parameters of PET remains unchanged while the crystals turn to better crystal lattice,greater crystal size and more regular folding.We also found that there is a slight reduction in crystal size between the two melting peaks,and an explanation is suggested for this phenomenon.