Al 2 O 3 nanoparticle-reinforced Al-12Si matrix composites were successfully fabricated by hot pressing and subsequent hot extrusion. The influence of weight fraction of Al 2 O 3 particles on the microstructure, mechanical properties, and the corresponding strengthening mechanisms were investigated in detail. The Al 2 O 3 particles are uniformly distributed in the matrix, when 2 and 5 wt. % of Al 2 O 3 particles were added to the Al-12Si matrix. Significant agglomeration can be found in composites with 10 wt. % addition of Al 2 O 3 nanoparticles. The maximum hardness, the yield strength, and tensile strength were obtained for the composite with 5 wt. % Al 2 O 3 addition, which showed an increase of about~11%, 23%, and 26%, respectively, compared with the Al-12Si matrix. Meanwhile, the elongation increased to about~30%. The contribution of different mechanisms including Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening were analyzed. It was shown that the thermal mismatch strengthening has a more significant contribution to strengthening these composites than the Orowan and load transfer strengthening mechanisms.