Melting point and heat of fusion of PET and its copolymers are studied.Theresults show that:(1)at temperature below its equilibrium melting point

PETmelt can crystallize and form crystals with high melting point.Equilibrium meltingpoint can be measured by relations of T_c and melting points or relations of mel-ting points of copolyesters and percentage of the third component;(2)heat of fu-sion of PET copolymers varys with the molecular structure of additive third com-ponent and its amount.Therfore

Flory’s theory must be corrected

since it didnot take accout of the effect of molecular structure of the third component on themolecular interreaction of polyester.This effect can be characterized by the dif-ferentia of heat of fusion of pure PET and that of copolyester when extraprola-ted to zero fraction of the third component.