This study was conducted to investigate the effect of Al2O3 on characteristics of A356 reinforced composites. The addition of Al2O3 as reinforced varied from 2 vf-% to 15 vf-% through Stir Casting methods to obtain the optimum addition Al2O3 as reinforced in mechanical properties of composite A356 as matrix and Al2O3 as reinforcement. Magnesium with of 10 wt-% was added to improve wettability between Al2O3 particles and Al A356. Addition of Magnesium is expected to form the spinel phase MgAl2O4 in interface area between Al2O3 and Al A356.The optimum tensile strength was found at 2 vf-%Al2O3 with value 131.55 Mpa. However, the value is still lower than the tensile strength of Al A356 As cast. It is caused due to the porosity and the agglomeration of Al2O3 were found in the microstructure of the composite Al A356/Al2O3. The Hardness increased from material monolithic Al A356 as cast, reached 37.43 HRB.