The effect of ultrasonic vibration (USV) on solidification microstructure of Al-11.5Si-4Cu-2Ni-1Mg-0.45Fe (wt%) piston alloy was investigated. Results show that the significant refinement and morphology transformation from strip into compact polygon can be observed for the primary Si in the alloys produced by USV. The enhanced heterogeneous nucleation and shortened growth time caused by the large undercooling which is triggered by cavitation are responsible for the refinement of primary Si. The morphology of eutectic Si gradually develops from long plate into a short plate structure with increasing the ultrasonic power. Meanwhile, the morphology of α-Al₅FeSi phases changes from coarsening block shape to half circle. The morphology evolution and refinement of eutectic Si as well as intermetallic particles are mainly due to the fragmentation of solid crystals, homogeneous solute distribution field and temperature field, and shortened eutectic growth time caused by cavitation.