Viscoelastic behavior of the non-Hooke deformation of amorphous PET film before yield was investigated in thetemperature region 74--80.5℃ around the glass transition temperature. The film specimen was drawn to yield point followedby unloading to zero stress

then the residual deformation was held constant

while the subsequent evolution of the stress wasrecorded. An induction period was found in the course of stress evolution fol1owed by a stress step-increase. The induction period decreases with increasing drawing temperature with an activation energy of 1.10 MJ/mol·K

which is attributed to the time needed for the relaxation of rubbery deformation through cooperative internal rotations.At temperatures lower than 74℃

there is no stress increase or the induction period becomes too long to be observed.Thus the nature of anelasticity in the non-Hooke region before yielding is attributed to stress induced rubbery deformation.The experimental results are interpreted in terms of Perez'rheological model of a scries connected Hooke spring and a Voigt element consisting of a parallel connected elastic spring and a dashpot.