Conventional friction stir processing (FSP) and ultrasonic-assisted friction stir processing (Ua-FSP) were applied to modify the microstructure and enhance the mechanical properties of as-cast A356 aluminium alloy. A series of experiments with different processing parameters were conducted in these two processing methods. Forming characteristics, grain-refinement, and mechanical properties in FSP and Ua-FSP were compared to determine the effect of ultrasound. The results revealed that ultrasound improved the grain-refinement level in both grain size and grain uniformity. Further, the effect of ultrasound on microstructural evolution was more significant with relatively high processing heat-input. Ultrasonic vibration also positively affected the hardness, tensile strength, and elongation. Graphic Abstract