The orientation of the noncrystalline (amorphous) phase in poly (ethylene terephthalate) fibers is analyzed by separating the amorphous scattering in the wide-angle x-ray diffraction patterns into isotropic and anisotropic components. Two parameters are used to characterize the amorphous orientation-the fraction of the anisotropic component and its degree of orientation. The x-ray amorphous orientation parameters are compared with the sonic modulus and the birefringence values. Our results illustrate that the intrinsic birefringence of the amorphous phase is not "intrinsic" but depends on its density. The role of the oriented amorphous phase in determining the strength (tenacity) and the dimensional stability (shrinkage) of the fibers is discussed. We conclude that although amorphous orientation determines the shrinkage, other factors such as the connectivity between the amorphous phase and the crystalline regions play an important role in determining the tenacity of the fibers.