In this study, microstructure and wear properties of A356 aluminum matrix nanocomposites reinforced with nano-Al2O3 particles were investigated. The powder metallurgy method was used for the production of 1 wt% and 2 wt% nano-Al2O3 particle reinforced nanocomposites. After 1 h of mechanical milling of A356 and nano-Al2O3 powders, green compacts were obtained by cold pressing. Green compacts were sintered at 550 °C in a vacuum environment (10−6 mbar) for 1 h. Samples were characterized by density, hardness measurements, scanning electron microscopy investigations, and wear tests. As the reinforcement ratio increased, there was a decrease in the densities of the nanocomposites, as well as an increase in the porosity. The highest hardness and the lowest weight loss values were obtained in 1 wt% Al2O3 reinforced nanocomposites. A decrease in hardness was measured at 2 wt% Al2O3 reinforced nanocomposites.