Stress-relaxation studies between room temperature and 140°C were made on oriented multifilament poly(ethy1ene terephthalate) (PET) yarn and on four other samples which had been prepared by heat-setting this yarn at 180 and 230°C while free to relax and when held taut at constant length. Using the time-temperature superposition principle, the data were superimposed, and the master curves of relaxation modulus were constructed for each sample; the relaxation-time spectra for these samples were computed using the Alfrey approximation. The results revealed that (i) the free-annealed samples show higher visco-elasticity than the corresponding taut-annealed samples and (ii) the relaxation-time spectra show two prominent relaxation processes on logarithmic time scale in the case of all the samples; the two relaxations being more intense in the free-annealed samples. These observations are interpreted in terms of fiber structure and morphology, which had been studied earlier; the amorphous orientation in the free-annealed samples is low, and they have a more distinct separation of the crystalline and amorphous phases. These factors facilitate molecular relaxation processes in the noncrystalline phase; the relaxations are therefore relatively more intense in these samples, and they exhibit more viscoelasticity.