It is very difficult to simultaneously refine and modify Si particles in hypereutectic Al-Si-Cu alloys to enhance their ductility. This study investigates how nanoparticles affect Si particles during solidification in hypereutectic Al-Si-Cu alloys. 0.5 wt% gamma-Al2O3 nanoparticles were added in hypereutectic Al-20Si-4.5Cu alloy melt and further dispersed through an ultrasonic-cavitation-based technique. The as-cast Al-20Si-4.5Cu-Al2O3 nanocomposites showed marked enhancements in both ductility and strength. The ductility of Al2O3 nanocomposite was more than two times higher than that of the monolithic alloy without the nanoparticles. Microstructural analysis with optical and scanning electron microscopy revealed that both the primary and eutectic Si particles were significantly refined. The primary Si particles were refined from star shapes to polygon or blocky shapes, and their edges and corners were much smoother. The large plate eutectic Si particles were also modified into the fine coralline-like ones. The porosity of alloy was also reduced with the addition of gamma-Al2O3 nanoparticles. Study suggests that gamma-Al2O3 nanoparticles simultaneously refine and modify Si particles as well as reduce porosity in cast Al-20Si-4.5Cu, resulting in unusual ductility enhancement that could have great potential for numerous applications.