The effects of drawing temperature and draw ratio on the contraction and extension of oriented amorphous PET film during thermal treatment has been studied. It has been shown that considerable thermal contraction of the oriented PET film before crystallization is due to relaxation of molecular chains in global sense while the spontaneous extension is related to crystallization of the oriented PET film. The maximum amount of contraction of the oriented PET film during thermal treatment increased with increasing λfor λ>2.3

owing to faster crystallization rate of PET film in highly oriented state. The maximum extension exibits similar behavior as that of maximum contraction with increasing λ and showed a peak at λ≌ 3.3

which isalmost the same draw ratio above which the densityof PET film began to increase during drawing.