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 widespread applications in the aerospace and automotive industries. The purpose of this study is to determine the influence of different amounts of La on the microstructures and mechanical properties of A356 alloys. The microstructures of as-cast samples were examined by using optical microscopy (OM), scanning electron microscopy (SEM) and image analysis. The mechanical properties were investigated by tensile test, and the quality index (Q = UTS + 150 × log (elongation)) was used to evaluate the modification efficiency of different La contents. The findings indicated that the modification efficiency in microstructures and mechanical properties of A356 alloy with 1.0 wt.% La are similar to those modified by the commercial modifier, 0.01 wt.% Sr. Furthermore, the results of thermal analysis reveal that there is no direct relationship between eutectic growth temperature and silicon morphology/modification rating.