paper report the use of multiwalled carbon nanotubes to reinforce and toughen gel-spun ultra high molecular weight polyethene fibers by adding 5 wt% ultra strong fibers with tensile strengths of 4.2 GPa and strain at break of 5% can be produced In comparison with the pure fiber at the same draw ratios these values represent increases of 18.8% in tensile strength and 15.4% in ductility In addition 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 and micro-Raman spectroscopy Carbon nanotube alinment along the tensil draw direction has been observed at high elongation ratios Such alignment induces strong interfacial load teansfer 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 mixture if the CNT alignment can be future optimixed