Friction stir process (FSP) was employed to produce nanocomposite surface layers on mild steel as the substrate. As the reinforcement material, nano-sized Al2O3 particles were introduced to the stir zone through a premade groove. The sampleswere characterized using optical microscopy, scanning electronmicroscopy, microhardness and unidirectional sliding pin-on-disk tests. FSP produced a composite layer on the surface consisting of ultrafine grains and the dispersed Al2O3 nanoparticles. In this process, second-phase particles were found to retard the grain growth through pinning the grain boundaries. The fabricated surface composite showed hardness of about three times higher than the original mild steel substrate. Furthermore, the as-received and the FSPed samples exhibited different wear behaviors and friction coefficients which were meticulously discussed based on the obtained results and observations. All in all, superior wear behavior of the FSPed samples was attributed to their highermicrohardness values originated fromthe finermicrostructure and the uniformly dispersed Al2O3 nanoparticles.