Polyethylene terephthalate (PET) melt-spun fibers were modified with multiwall carbon nanotubes (MWCNT) to obtain conductive microfibers smaller than 90 μm in diameter. Physical properties such as crystallinity and orientation of as-spun fibers were studied by X-ray diffraction, Raman spectroscopy, and microscopy techniques at different draw ratios (DR) and MWCNT concentrations. Morphological and orientation analysis of MWCNT after melt-spinning process showed agglomerates formation and highly oriented CNTs. The study of the orientation of PET crystalline phase in drawn fibers proved that the addition of nanoparticles decreases the orientation of crystalline units inside the fibers. The orientation of MWCNT as well as that of PET chains was studied using Raman spectroscopy at different DR and a high degree of CNT orientation was observed under high DR conditions. Mechanical and electrical properties of as-spun fibers were also investigated. Our results showed that it was possible to achieve conductive fibers at a MWCNT concentration of 2% w/w, and more conductive fibers using higher DR were also obtained without increasing the MWCNT concentration. Mechanical properties results showed interestingly high value of maximum tensile strain at break (εmax) of nanocomposite fibers, up to three times more than pure PET fibers. POLYM. ENG. SCI., 50:1956–1968, 2010. © 2010 Society of Plastics Engineers