A356 hypoeutectic alloy is a heat-treatable Al–Si–Mg system with a normal composition of Al–7 wt.% Si and about 0.35 wt.% Mg which has seen widespread applications in the aerospace and automotive industries. The purpose of this study is to determine the influence of different amounts of Ce on the microstructures and mechanical properties of A356 alloys. The findings indicate that modification efficiency in microstructures and mechanical properties of A356 alloy are greatly enhanced by adding 1.0 wt.% Ce. Two kinds of intermetallic compounds are found in the study, including Ce–23Al–22Si and Al–17Ce–12Ti–2Si–2 Mg (in wt.%) phases. Moreover, though the ductility of the Ce modified alloys are both greatly improved, there is no positive effect on the ultimate tensile strength that can be attributed to the formation of an Al–17Ce–12Ti–2Si–2 Mg phase. Furthermore, the results of thermal analysis reveal that there is no direct relationship between eutectic growth temperature and silicon morphology/modification rating.