The modification effects of alloying element Sb and heat treatment on Al-15.5wt%Mg2Si alloy were investigated by Olympus microscopy (OM), scanning electron microscopy and energy disperse spectroscopy (SEM-EDS), and X-ray diffraction (XRD). It is found that Sb plays a significant role in shaping primary Mg2Si phase and eutectic Mg2Si phase in Al-15.5wt%Mg2Si alloy. The Sb addition of about 1.0wt% makes the resultant alloy show the finest primary Mg2Si phase and the eutectic Mg2Si phase with well distribution. But further increasing the Sb content decreases the amount of primary Mg2Si phase, and some segregated phases appear at regions between the grains. In addition, heat treatment can modify the microstructural feature of Sb-modified Al-15.5wt%Mg2Si alloy in terms of obviously shortening the nodulizing time of primary Mg2Si phase and eutectic Mg2Si phase. The modification effects of alloying element Sb and heat treatment on Al-15.5 wt% Mg2Si alloy were investigated by Olympus microscopy (OM), scanning electron microscopy and energy disperse spectroscopy (SEM-EDS), and X-ray diffraction (XRD) is found that Sb plays a significant role in shaping primary fine Mg2Si phase and eutectic Mg2Si phase in Al-15.5wt% Mg2Si alloy. The Sb addition of about 1.0wt% makes the resultant alloy show the finest primary Mg2Si phase and the eutectic Mg2Si phase with well distribution. But further increase the Sb content decreased the amount of primary Mg2Si phase, and some segregated phases appear at regions between the grains. In addition, heat treatment can modify the microstructural feature of Sb-modified Al-15.5 wt% Mg2Si alloy in terms of obviously shortening the nodulizing time of primary Mg2Si phase and eutectic Mg2Si phase.