We analyzed mechanical properties and structure of polypropylene fibers with different concentrations of single-wall carbon nanotubes (SWNTs) and draw down ratios (DDR). Tensile tests show a three times increase in the Young's modulus with addition of only 1 wt% SWNT, and much diminished increase of modulus with further increase in SWNT concentration. Microscopic study of the mechanism of reinforcement by SWNT included Raman spectroscopy and wide-angle X-ray diffraction (WAXD). The results show linear transfer of the applied stress from the polymer matrix to SWNT. Analysis of WAXD data demonstrates formation of a beta-crystal phase in polypropylene matrix under the strain. (C) 2004 Elsevier Ltd. All rights reserved.