Our tribological experiments on nanocrystalline (nc) Ni (grain size down to approximately 10 nm) showed significant reductions in both, the coefficient of friction and wear rate compared to its microcrystalline (mc) counterpart. A consistent relationship was found between grain size, hardness and tribological behavior. In the present study, the wear mechanism was investigated by conducting transmission electron microscopy (TEM) and nanoindentation experiments in the wear track region. TEM observations revealed that sliding wear developed two entirely different substructures in mc and nc Ni. Under the extensive plastic deformation, surface nanocrystallization occurred in the former and deformation-induced grain growth in the latter. These changes were consistent with the nanoin-dentation measurements from the wear track. Hardness in the mc Ni was increased due to work hardening/surface nanocrystallization. On the contrary, hardness remained at similar or slightly lower levels for nc Ni probably due to grain coarsening from the activation of grain boundary-related modes of deformation. The two different deformation mechanisms are consistent with the observed differences in frictional behavior and wear resistance that involves wear/fatigue for mc Ni and fine scale abrasion for nc Ni.