In order to investigate the fine structural changes of poly (ethylene terephthalate) film by hydrazinolysis, PET was treated with hydrazine hydrate in methanol at $30^{circ}C$ for various times. Initially apparent crystallite size and degree of orientation are increased and then, gradually decreased with hydrazinolysis. According to thermoluminescence measurement, trap site decreased by dissolution and then gradually increased with growth of defect in structure. Maximum peak temperature of tan and dynamic loss modulus E" upon hydrazinolysis shifted to the lower temperature in the progress of hydrazinolysis.ysis.