The structure/property relationships in a series of melt-spun poly(ethylene terephthalate) (PET) fibers processed under different conditions have been investigated. The structure of the fibers was characterized using a variety of techniques, including gel permeation chromatography (GPC), differential scanning calorimetry (DSC), optical birefringence, and wide-angle x-ray scattering (WAXS). The mechanical properties of the PET fibers were also investigated, and the fibers showed different stress-strain and modulus- strain curves as a result of the different processing conditions employed. Raman spectroscopy was used to study molecular deformation processes in the PET fibers. It was found that most of the PET Raman bands shifted to a lower wavenumber when the fibers were subjected to axial tension, and the shift of the 1616 cm−1band was examined in detail. It was found that there was a linear relationship between the position of the center of gravity of the band and stress and that the stress-induced band shift factor of −4].0 cm−1/GPa was identical for all PET fibers studied. Band broadening was also found under stress, and the Raman bandwidth (full width at half maximum, FWHM) provided information about the spread of molecular stresses resulting from the application of macroscopic tensile stress to the fibers. The molecular deformation processes in the PET fibers have been interpreted in terms of a modified series aggregate model in which the molecules are subjected to a constant average level of stress.