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Abstract: Ultra high molecular weight polyethylene (UHMWPE) is a polymer that is widely used in industrial and orthopaedic applications. In this work, pristine multiwalled carbon nanotubes (MWCNTs) were incorporated into UHMWPE in different concentrations (1, 3 and 5wt.%) using a ball milling process. UHMWPE/MWCNT nanocomposites were gamma irradiated at 90kGy to improve the interaction between MWCNTs and the polymer matrix. Structural, thermal and mechanical characterizations were conducted by means of transmission electron microscopy (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and uniaxial tensile techniques. Gamma irradiation produced an increase in the melting temperature, crystallinity and temperature of maximum decomposition rate. The irradiation produced a 38% decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. An increase in thermal stability was also observed for the nanocomposites compared to neat UHMWPE. The tensile tests showed a 38% increase in the Young’s modulus in the reinforced nanocomposites and a small decrease in toughness (5%). Gamma irradiation of the nanocomposites increased crystallinity, which was related to an increased lamellar thickness, and also improved their thermal stability. The Young’s modulus increased by up to 71% for irradiated nanocomposites and their toughness showed no significant changes in comparison with the non-irradiated nanocomposites. The incorporation of MWCNTs reduced the negative effects of irradiation and compensated for the reduction in toughness. This fact might be attributed to the radical scavenger behaviour of the MWNT as was proved by Electron Spin Resonance (ESR) detection of the radiation-induced radicals. [ABSTRACT FROM AUTHOR]