The effect of Sc- and Zr-addition and annealing treatment on the microstructure and properties of as-cast Al-Si alloys were investigated by optical microscopy, transmission electron microscope, scanning electron microscope, microhardness and tensile test. The results show that the addition of Sc and Zr can significantly improve the mechanical properties of the Al-5.5Si alloy, as a consequence the hardness, tensile strength and yield strength of the alloy increased by 33%, 38% and 52% respectively, while the elongation kept basically unchanged. The addition of Sc and Zr in the Al-5.5Si alloy reduced the average grain size of α-Al from 203 μm to 130 μm, and a large amount of Al₃ (Sc_1-xZr_x) nanoparticles (10~15 nm) precipitated in the α-Al matrix which can significantly increase the stacking faults or micro twinning density of eutectic Si. It is noted that the annealing temperature has a greater impact on the properties of the as-cast alloy: the hardness of the alloy had an increasing trend when annealing at lower temperature (below 160℃), and the hardness of the alloy had a decreasing trend when annealing at higher temperature (above 280℃). The significant downward trend can be attributed to the secondary precipitation of nano-Si phase.