This paper reports the use of multiwalled carbon nanotubes (MWCNT) to reinforce and toughen gel-spun ultra high molecular weight polyethylene (UHMWPE) fibers. By adding 5 wt% MWCNT, ultra strong fibers with tensile strengths of 4.2 GP and strain at break of similar to 5% can be produced. In comparison with the pure UHMWPE fiber at the same draw ratios, these values represent increases of 18.8% in tensile strength and 15.4% in ductility. In addition, a 44.2% increase in energy to fracture has also been observed. The mechanism of reinforcement has been studied using a combination of high resolution scanning electron microscopy (SEM) and micro-Raman spectroscopy. Carbon nanotube alignment along the tensile draw direction has been observed at high elongation ratios. Such alignment induces strong interfacial load transfer both at small and large strains to enhance the stiffness and tensile strength of the composite fiber. Consequently, the mechanical properties of the composite fiber follow closely with the rule of mixtures. Our work also reveals potential for positive deviation from rule of mixtures if the CNT alignment can be further optimized. (c) 2006 Elsevier Ltd. All rights reserved.