The effect of ion implantation including ion species (N2+ and C3H8+) and the fluences (1×1014–5×1015 ions/cm2) on the surface energy of ultrahigh molecular weight polyethylene (UHMWPE) were investigated. The total surface energy increases significantly after implanting with the fluence of 1×1014 ions/cm2 regardless of ion species, then, the total surface energy slightly increases for N2+ implanted UHMWPE and decreases slightly for C3H8+ implanted UHMWPE with a further increase of fluence. The structural changes of UHMWPE with different fluence for different ion species are very similar. The linear chains of UHMWPE are damaged and cross linking is generated after implantation. As the fluence increases, the polymer surface becomes more disordered, and the surface becomes hydrogenated amorphous carbon when the fluence exceeds 1×1015 ions/cm2. The surface roughness increases with the increase of the fluence regardless of ion implantation species. © 2003 American Institute of Physics.