The influences of ultrasonic vibration (USV) treatment on the refining effects of primary Si particles and acicular five-element compounds with rich RE in high silicon Al-Si alloy with complex modification of P and RE were studied. The primary Si particles in this alloy was about 20 μm in size under direct USV for 2 min in the semi-solid temperature range, compared to about 30 μm in the alloy without USV. The primary Si particles in the former distributed more homogeneously and in regular shape, but had lower volume fraction of primary Si than that of the alloy without USV for the solid solubility of Si in α-Al matrix increased with USV. The eutectic Si of this alloy was modified moderately with 0.6% RE and long acicular compounds with rich RE formed which decrease the mechanical properties of alloy for the effect of cutting the matrix. Short rod-like compounds with rich RE about 10~15 μm in length were obtained with the combined effects of USV and T6 heat treatment.