Effects of physical ageing on the yield behavior during uni-axial stretching of amorphous PET film were studied by differential scanning calorimetry and stress-strain measurements. It was shown that the endothermic peak at Tg

the yield stress

and the stress yield peak in the stress-strain curve all increased as physical ageing progressed. On passing through the yield point

DSC indicated that theendothechc peak disappeared. Post-yield amorphous film followed by further heat treatngllt below Tg

a yield stress peak in the stress-strain curve was shown again during re-stretching.The nature of the physical ageing and stress yield were explained on the basis of cohesional entanglements.