A method of adding graphene oxide (GO) and further reinforcing by the treatment of gamma-irradiation was used to enhance the properties of ultrahigh molecular weight polyethylene (UHMWPE). The effects of GO and irradiation on UHMWPE, particularly the influence of irradiation on UHMWPE/GO nanocomposites, were investigated from the aspects of thermal and mechanical properties. The results show that the thermal stability of UHMWPE/GO nanocomposites was slightly decreased by irradiation. But both GO doping and irradiation increased the crystallinity of UHMWPE. A similar variation tendency was appeared between compressive properties and crystallinity. A postulated mechanism was established to explain how GO doping and irradiation actually improved the properties of UHMWPE.