Abstract: In situ Al/Mg₂Si/Si composites were fabricated by using simple weight cast technique. Effects of extra Si contents on microstructure of Al/Mg₂Si composite were investigated by optical microscopy and SEM with an EDXA accessory. Al Mg₂Si Si ternary diagram was also introduced to analyze the microstructure. Experimental results indicated that extra Si in Al/Mg₂Si system produced sphere like α Al grains and ternary eutectics with low aspect ratio. These microstructure improvements resulted in a significant increase in the materials' tensile properties.