In order to investigate the fiber structure development and the fiber properties of poly (ethylene terephthalate)(PET) in high-speed spinning, PET polymer (1.V.=0.62 dl/g) was spun at the take-up velocities of 1-7 km/min. The changes of the structure and property caused by various take-up velocities were studied by measuring density, bireflingence, X-ray diffraction, DSC, TMA, dynamic viscoelasticity, and tensile behavior. The PET fibers spun at high take-up velo-cities show very different structure formation and property compared to the regular PET fiber. The orientation, especially in crystalline phase, is developed with the increase of take-up velocity. Orientation-induced crystallization behavior of PET fibers in high-speed spinning was appeared at take-up velocity about 4-5 km/min. As the take-up velocity increases, the amorphous orientation factor also increased, but the dynamic loss tangent 6 decreased. There was a linear relation between microscopic density calculated by Lorentz-Lorenz equation and experimental density.