The dynamic mechanical properties of polyethylerie terephthalate(PET)monofilament samples with different crystallinity and draw ratio were determined over 100 t0 430°K using the vibrating reed method．In this temperature range two internal friction peaks were observed at 243°K(βc)and 354--383°K(αa)．The βc and αa peaks were shown to be influenced by the variation of crystallinity． From the view point of Takayanagi’s mechanical model of crystalline polymers，the β absorption may be attributed to the local twisting motion of the molecular chains in the frozen noncrystalline phase and in the de-fective region within the crystalline phase，and the αa absorption to the translational mo-tion of the segments in the noncrystalline region．The temperature of the αa peak increa3es with increasing draw ratio，while the βc peak remains unchanged．This suggests that the stretching of PET fibre would mainly change the interchain interactions in the noncrystalline region．Takayanagi’s method of determining the crystallinity of PET samples from the loss moduli of βc and αa peaks was confirmed for the fibre samples．