To investigate the mechanism of fiber structure development for poly(trimethylene terephthalate) (PTT) in high-speed spinning, the PTT fiber was spun with take-up speeds from 1 to 8 km/min and simultaneously birefringence and diameter in spin-line were measured by on-line measurement system. The orientation-induced crystallization of PTT fiber started to be developed at 3-4 km/min, where an abrupt decrease in diameter and an increase in birefringence appeared. The birefringence increased up to 4 km/min, decreased suddenly, and then increased gradually. The sudden decrease of birefringence at 4-5 km/min might be caused by an increase of crystalline fraction due to the fact that the intrinsic crystalline birefringence of PTT is over 10 times as low as that of PET. In WAXD images, crystalline diffraction emerged faintly at 3 km/min and distinct diffraction arcs were observed at 4-5 km/min and above. The diffraction intensity increased and the tilting angle also increased with take-up speed. The long period structure observed in SAXS pattern started to emerge at 6 km/min, and its scattering intensity increased with take-up speed. The long period structure was similar to 11-12 nm long. The cold crystallization temperature in DSC thermogram shifted to lower temperature and diminished due to the orientation-induced crystallization as take-up speed increased, but the melting temperature hardly increased unlike PBT and PET. (c) 2008 Wiley Periodicals, Inc.