In this study, A356 aluminum alloy matrixcomposites reinforced with different weight percentages of SiC nano- and microparticles respectively with 50 nm and 5 µm average particle sizes were fabricated by stir casting method. Due to the effect of T6 heat treatment on the strength and hardness of A356alloy, the obtained composites were subjected to the T6 heat treatment. The mechanical properties such as hardness and compressive properties of the composites were investigated. Microstructures of the samples were also investigated by an optical microscope (OM), scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM). Microstructural investigation indicated that T6 heat treatment led to thechange of eutectic silicon morphology and formation of theMg2Si precipitates during age hardening stage, leading to increase the hardness and compressive strength. The results showed that an increase in wt.% of nanoparticles led to an increase in hardness and compressive strength. The results of microstructural investigationshowedthe relatively uniform distribution of reinforcement particles. Also, the strength and hardness of the composites reinforced with nanoparticleswere greater than those of the composite reinforced with microparticles, even with higher weight percent of reinforcement particles. Hardness and compressive strength at 35% strain for the composite reinforced with 1.5 wt.% nanoparticles were respectively obtained 62 HBN and 252MPa, which are improved compared to the base alloy.