In the present study, the tested hypereutectic Al-21 wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modif ication effects and mechanism of P+Ce complex modif ier on the Si phase of hypereutectic Al-21 wt.%Si alloy were studied, and the aging precipitation behavior after modif ication was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles are signif icantly ref ined after modif ication, while the needle-like eutectic silicon crystals become f ibrous and short. It was found that the mechanism of phosphorus modifi cation on the primary silicon can be attributed to heterogeneous nucleation of Al P, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modif ied hypereutectic Al-21 wt.%Si alloy, there existed some strengthening phases such as Al4Cu9, Al2 Cu, AlC u3, and Al57Mn12. The P+Ce complex modif ier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21 wt.%Si alloy was modif ied using 0.08%wt.P + 0.6wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and elevated temperatures are optimized(Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ℃). In the present study, the tested hypereutectic Al-21 wt.% Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modification effects and mechanism of P + Ce complex modifier on the Si phase of hypereutectic Al-21 wt.% Si alloy were studied, and the aging precipitation behavior after modification was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles were signif icantly refined after modification, while the needle-like eutectic silicon crystals became f ibrous and short. It was found that the mechanism of phosphorus modifi cation on the primary silicon can be attributed to heterogeneous nucleation of Al P, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modified hypereutectic Al-21 wt.% Si alloy, there existed some strengthening phases such as Al4C The P + Ce complex modif ier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21 wt.% Si alloy was modified using 0.08% wt. P + 0.6 wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and an elevated temperatures are optimized (Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ° C).