Coarse primary Mg2Si was usually found in Al-Mg-Si alloys. In this experiment, to refine the coarse primary Mg2Si size, ultrasound was used as a new process. Ultrasound was injected to the molten Al-Mg-Si melts directly and its effect was significant. Average size of primary Mg2Si was reduced from 70㎛ to 15㎛, morphology was also changed to polygonal from irregular shape. Moreover tensile strength was improved by 30%, from 112 to 148 after ultrasonic injection.