The study purpose is to investigate effects of ultrasonic radiation on refinement of primary Si crystals of hypereutectic Al–Si alloys and to examine the refining effect in die casting. The ultrasonic refining effect was examined by using Al–Si alloys without or with addition of phosphorus. The experiments included melting of alloy in a crucible, radiation of ultrasound into the melt at temperatures at least 60 K higher than the Si crystallization temperature, holding of the melt for 20–80 s and pouring of it into a Cu mold by gravity casting. The refining effect was found to be dependent on P content and holding time. The EPMA investigation revealed that the ultrasonic radiation increases the number of AlP particles acting as heterogeneous sites for the primary Si nucleation. The refining effect was significant at the holding time of 20–50 s. Then, based on the above results, the radiation conditions for the die casting were determined. A key point of the conditions was that the refining effect and melt fluidity are kept at the desired level for a period after the radiation. Microstructural investigation showed considerable refinement of primary Si crystals and Al–Fe–Mn–Si compounds. The ultrasonically treated die castings exhibited an improved tensile strength.