A systematic study of the effect of physical ageing and cooling rate on a-relaxation peak are carried out in PVC. It was said that a-peak and Tg are not the same concept. Both are closely connected with amorphous state, but show different behaviours in the same pre-treatment conditions: varying physical ageing time in sub- Tg or changing cooling rate from Tg + 20℃ to room temperature . Some interesting results were obtained as follows:When Tg lowers with increacing ageing time or decreacing cooling rate, a-peak temperature (Tα) keeps constant. On the other hand, the intensity of the a-peak decreases. This behaviour shows that a- relaxation peak is not a typical linear anelastic internal friction peak, as there is a quenching peak added on it.A detail discussion will be given in this paper.