In this study, the microstructure evolution and mechanical properties of hypereutectic Al-20Si alloy containing rare earth Yb and Al-5Ti-1B refiner were investigated. The microstructures of the samples were characterized by optical microscopy, scanning electron microscopy, and electron probe microanalysis. Microstructural analysis demonstrated that the primary Si was significantly refined from a coarse polygonal, platelet-like or star-like shape into a fine blocky shape; the eutectic Si structure was modified from a coarse platelet-like/needle-like structure into fine particles or a fibrous structure; and the coarse α-Al dendrites were refined into fine equiaxed dendrites after the addition of 0.5% Yb and 0.3% Al-5Ti-1B complex. However, the primary and eutectic Si phases became coarser when the content of Al-5Ti-1B refiner reached 0.4%. The mechanical properties of the Al-20Si alloy with different modifiers and different modifier concentrations were investigated with the tensile test. The results showed that the ultimate tensile strength and elongation of the prepared sample increased by 83.7% and 92.1%, respectively.