Ultra-high molecular weight polyethylene (UHMWPE) was surface treated by nitrogen plasma immersion ion implantation (PIII), with the main aim of improving its wear resistance. Accelerating voltages (U) between 15 and 30 kV, fluences (F) between 1×1017 and 3×1017 cm-2 and fluence rates (FR) between 3×1013 and 7×1013 cm-2 s-1 have been applied. XPS was used to characterise the surface chemical composition and structure. Changes induced in the surface mechanical properties like hardness (H), reduced modulus (E) and in the tribological property of volume loss upon uniform wear test (V) were studied by nanoindentation and multipass wear measurements. The evolution of surface topography was followed by measuring the mean roughness (Ra). The macroscopic temperature (T) developed during the PIII-treatment was also studied. Incorporation of N and O took place into the surface layer. With the increase of U the surface N-content tended to decrease. The bulk plasmon loss energy of the C 1s peak increased from 20 eV up to about 25 eV, suggesting densification and the formation of amorphous hydrogenated carbon nitride-like layer. H, T and Ra increased, and V decreased upon PIII treatment, while E either decreased or increased depending on the actual process parameter set applied. In the parameter range studied Hmax, Emax and Ra,max values have been observed at Umax, Fmax and FRmin. Vmin and Tmax have been observed at Umax, Fmin and FRmax, suggesting that the thermal effect is a dominant factor in determining the extent of reduction in the wear rate.