Poly (trimethylene terephthalate) (PTT) is a semicrystalline polymer that has many of the same advantages as its polyester cousin poly(ethylene terephthalate)(PET), but fibers made from PTT provide better resilience and elastic recovery than PET. In present study, we aim to investigate the effect of heat treatment under alternating tension on the fiber structure, which is associated with the fiber elasticity. Commercially available PTT fiber was selected to be heat-treated at the temperature 140oC-160oC under alternating tension with 1-3 cycles, and each cycle continued 60 min including 30 min with tension and 30min without. The results show that the heat treatment has almost little effect on the crystallization of PTT fibers when the treatment temperature is 140 oC, while cold crystallization peaks appear in DSC curves for the fibers when the temperature increases to 150 oC or 160 oC under suitable tension (less than 6cN/tex). The crystallinity increases and the grain size decreases with the increase of the heat treatment cycles. FTIR characteristic peak intensity of PTT fibers around 1422 cm-1 and 1040 cm-1 with the increase of the temperature and cycles changes, which means the transformation of conformation of propandiol segment in the PTT chain. No yield point appears within the elastic strain less than 15% in cyclic drawing curves of the PTT fibers after heat-treating, indicating that irreversible polymer segment motion did not occur.