The hardness and tensile behaviors of aluminum matrix composites reinforced with nanometric Al2O3 particulate have been found to increase with the volume fraction of the reinforcement. Above 4 vol.% of Al2O3, however, the strengthening effect leveled off because of the clustering of Al2O3. The main strengthening mechanism was Orowan strengthening of effective nano-Al2O3 particles evenly distributed in the matrix. (C) 2004 Elsevier B.V. All rights reserved.