In this study, the effect of the carbon nanotube (CNT) aspect ratio and surface functionalization on the mechanical behavior and morphological changes of polyamide (PA)-based fibers was investigated. Composites were prepared by the melt blending of CNTs with PA, and at a later time, the fibers were prepared by melt spinning and cold drawing. A reinforcement effect was noticed for all of the CNTs samples, and the increase in the mechanical properties and dimensional stability was more pronounced for highly oriented filaments. When the elongational flow was increased, the orientation of CNTs along the fiber direction was observed, but the nanotube alignment was much more difficult for CNTs with ultrathin outer diameters because of nanotube waviness and folding. Moreover, the presence of functional groups on the CNT surface hindered their orientation along the fiber direction because some interaction between the functional groups could occur. The morphological variations of the oriented, anisotropic fibers, as studied with transmission electron microscopy, scanning electron microscopy, small-angle X-ray scattering, and differential scanning calorimetry analysis, were correlated with changes in the mechanical behavior. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013