The effect of alumina nanoparticles contents with the average size of 40 nm on the microstructure and wear behavior of A356/Al2O3 nanocomposite produced through stir casting was studied. The alumina nanoparticles with the amounts of 1, 1.5 and 2 weight percent, were incorporated into the molten matrix alloy being mechanically stirred, along with blowing an inert gas. To improve the nanoparticles wettability, 1wt% Mg was added to the molten matrix alloy. The microstructure of the nanocomposite was studied by using optical and scanning electron microscopes. The microstructural observations showed that the alumina nanoparticles have been evenly distributed within the matrix. The percent of porosity measured by the Archimedes method is increased with increasing the nanoparticles weight percent from 1.3% to 3.2 %. It was shown that, the increased percent of porosity can be attributed to the increase in the particles agglomeration with increasing particles weight percent. To study the wear behavior of the materials, nanocomposite containing 1wt% alumina, having the lowest porosity percent and even distribution of nanoparticles, was selected. The wear test showed that the addition of nanoparticles to the matrix alloy gave rise to enhance its wear resistance. Study of the worn surfaces and sub surfaces showed that the dominant wear mechanism for the materials used in the current work is probably an adhesive wear followed by the formation of a mechanically mixed layer (MML). However, this mechanism occurred with the less wear rate for the nanocomposite as a result of the presence of reinforcing particles.