Ultrasonic waves (frequency 20 kHz and maximum intensity 400 W) were applied in molten Mg alloys during solidification to a cylinder 30 mm in diameter and 100 mm long. Applying this treatment to Mg-4Zn-4Sn-1Si resulted in refinement of the dendritic structure and more uniform dispersion of the Mg 2 Sn and Mg 2 Si precipitates. The same treatment carried out on Mg-1 . 5Ca-3Zn alloy resulted in transition from dendritic solidification to equiaxed structure with a relatively uniform grain size. In addition, the formation of Mg 2 Ca was significantly suppressed.