The influence of nanoscale reinforcement on the mechanical behavior of ultrafine-grained composites was studied. Al 5083 (Al–4.5Mg–0.57Mn–0.25Fe) composites, with grain size of 115nm and B 4 C reinforcement size of 38nm, were fabricated via cryomilling and consolidation. The result reveals that the presence of nanoparticles enhances strength by interacting with dislocations, while simultaneously retarding grain growth. Furthermore, the nanoparticles-reinforced composite exhibits enhanced plasticity relative to the same material reinforced with micrometric particles. The underlying mechanisms are discussed.