Grain refiner has conventionally been added during the casting process of grain refinement, but it has recently become a recycling problem, which calls for the efforts for development of a new grain refinement method. In this paper, a technological breakthrough has been achieved, where grain refinement was successfully completed by applying ultrasonic vibration during the solidification process of aluminum cast alloys. The refinement mechanism was clarified to determine the temperature range to apply the optimum vibration.