Acoustic emission during sub-Tg annealing fort amorphous polyethylene terephthalate (PET) sheet quenched from melt has been observed. The mechanical properties

texture and glass transition of annealed specimens have been studied. It is shown that some of PET chains change from non-equilibrium state to equilibrium state during sub-Tg annealing

which leads to stress concentration in specimen. When the level of internal stress approaches to the limit

the acoustic waves are emitted due to sudden releasing of stress and the micro-defects are for-med. This is one of the important reasons

the causes the loss of mechanical properties to an-nealed specimens.