Poly(trimethylene terephthalate)(PTT) fibers were heat-treated in relaxation, with tension or with alternating tension at 160 ℃. The conformation transition of PTT fibers before and after heat-treatment was studied by Fourier transform infrared spectroscopy. The elastic recoveries of PTT fibers before and after treatment were compared by elastic meter and dynamic thermomechanical analyzer. The results show that the higher-energy conformation of trans-conformation converts to the lower-energy conformation of gauche-conformation after alternating tension heat treatment, which increase the elasticity and its stability of the fibers. The elastic recovery of the fiber treated with alternating tension after 5 cyclic drawing at 30% fixed elongation is still 79%, being about 10% higher than that of the untreated PTT fiber.