In situ Mg2Si/AI composites are receiving increasing attention for industrial applications because of their inherently stable interfaces, light weight, excellent combination of mechanical properties and low processing costs. The composite is formed through in situ nucleation and growth of a reinforcing phase Mg2Si from the parent matrix during solidification. In this study, we report the effect of ultrasonic stirring with different times on the solidification structure and mechanical properties of in situ Mg2Si/Al composites. X-ray diffraction analysis, optical microscopy and scanning electron microscopy were used to analyze the microstructural evolution of the composites. The mechanical properties of the composites were tested by using hardness and tensile testers. Our results showed that 40 s ultrasonic stirring resulted in the optimal impact on the refining both the primary and eutectic Mg2Si particles and improving the shapes of the primary Mg2Si particles. The composites with 40 s ultrasonic stirring exhibited simultaneously enhanced tensile strength and elongation and the tensile fracture morphology was shown to be quasi-cleavage with a large number of dimples. This study proves that ultrasonic stirring is effective in degassing, removal of impurities, refining, and improving the shapes of the reinforcing phase, leading to significantly enhance the mechanical performance of the composites. (C) 2016 Elsevier B.V. All rights reserved.