The effect of Gd modification on the microstructure and mechanical properties of A356 aluminum alloys was investigated using the metallurgical microscopy, scanning electronic microscopy, X-ray diffraction and mechanical testing. The addition of 0.2 wt.%–0.4 wt.% Gd had an excellent refining effect on primary α-Al grains and a modification effect on Si phases in its as-cast state. The needle-like Si phases were adjusted into fine particles and uniformly distributed in the matrix by the T6 treatment, especially for the 0.2 wt.% Gd-modified alloy. The Gd additions introduced the Fe-containing Gd Al2Si2 compounds, which precipitated in the forms of flakes and bulks. The Gd Al2Si2 and β-Al5 Fe Si phases were also refined by the T6 treatment. The mechanical properties of the Gd-modified alloys were very poor in as-cast state; however, the highest strength and elongation were obtained for the 0.2 wt.% Gd-modified alloy by the T6 treatment. The effect of Gd modification on the microstructure and mechanical properties of A356 aluminum alloys was investigated using the metallurgical microscopy, scanning electronic microscopy, X-ray diffraction and mechanical testing. The addition of 0.2 wt.% - 0.4 wt.% Gd had an excellent refining effect on primary α-Al grains and a modification effect on Si phases in its as-cast state. The needle-like Si phases were adjusted into fine particles and uniformly distributed in the matrix by the T6 treatment, especially for the 0.2 wt. The Gd additions introduced the Fe-containing Gd Al2Si2 compounds, which precipitated in the forms of flakes and bulks. The Gd Al2Si2 and β-Al5 Fe Si phases were also refined by the T6 treatment. The mechanical properties of the Gd-modified alloys were very poor in as-cast state; however, the highest strength and elongation were obtained for 0.2 wt.% Gd-modified alloys by the T6 treatment.