Ultrasonic stirring treatment (UST) of molten metal has significant effects on the solidification microstructure of A356 alloy, which includes grain structure, distribution of inclusions, refinement of secondary phases, etc. The primary causes are due to ultrasonic cavitation, acoustic streaming and propagation of ultrasound waves in media. However, the mechanism of how those effects happen are not fully understood and quantified. In this research, molten A356 alloy was treated with high power ultrasound at a frequency of 18 kHz, and then at relatively high superheat, the melt was cast into a permanent metal mold which complies with ASTM B108-02. The UST processing system was custom built to perform the present UST study. The relatively high superheat condition is similar to the one used in the standard foundry practice, which will assist in the scale up of practical application of ultrasonic stirring technology. The selected parameters for the ultrasonic stirring technology (UST) were determined by using an UST modeling software tool that was recently developed and validated. The UST modeling software tool is capable of modeling and simulating the acoustic streaming and ultrasonic cavitation as well as the microstructure evolution during the solidification of cast alloys. Since the UST was preceded in the molten alloy, no dendrites are growing during the UST processing. Besides, more energy is required for homogeneous nucleation to occur. Consequently, the dominant mechanism of nucleation in this research is heterogeneous nucleation. The microstructure and mechanical properties of the A356 alloy processed with and without UST were analyzed and compared in detail in this study. It was demonstrated that the ultrasonically-stirred A356 alloy shows superior microstructure characteristics with very low micro-porosity levels and improved tensile properties when compared with the standard A356 alloy.