A co-rotating twin screw extruder with 3 zones of mixing elements was used to produce nanocomposite fibers of polyethylene terapthalate (PET) and carbon nanotubes. The PET and the CNT materials were fed using two separately controlled feeders. The polymer was fed at the first entrance to the extruder; whereas, the CNT was fed at three different positions located along the extruder barrel. The results showed that an addition of 1 wt% CNT enhanced the mechanical properties of PET/CNT nanocomposite fibers. The yield strength, tensile strength, and toughness were increased by 80%, 51%, and 134%, respectively, in contrast to the mechanical properties of pristine PET fibers. The scanning electron microscope (SEM) images showed that the feeding position with three mixing zones gives the most uniform distribution of CNT and the best spinnability of the PET/CNT fibers.