In　this　paper,　semi-solid　Al-7Si-0.4Mg-x　erbium(Er)　(x=0,　0.1,　0.3,　0.5,　0.8wt%)　alloys　are　fabricated　via　electromagnetic　stirring　method.　The　effects　of　Er　on　the　microstructures　and　mechanical　properties　of　semi-solid　Al-7Si-0.4Mg　alloys　are　investigated.　Results　show　that　the　eutectic　-phase　is　modified　to　fine　sized　Er-poor　-phase　and　primary　-Al　is　refined　in　semi-solid　Al-7Si-0.4Mg　alloys　with　the　addition　of　Er.　Meanwhile,　the　plate-like　shape　eutectic　Si　is　transformed　into　fibrous　eutectic　Si　with　addition　of　Er,　which　has　an　excellent　modification　effect　on　the　eutectic　Si.　Nucleation　rate　of　the　Er-poor　-phase　is　increased　with　a　higher　super　cooling　rate,　which　causes　the　refinement　of　primary　-Al　and　the　modification　of　eutectic　Si　phase.　With　the　addition　of　0.3wt%　Er,　semi-solid　Al-7Si-0.4Mg　alloys　obtain　the　best　tensile　properties.　The　ultimate　tensile　strength　and　elongation　values　are　228MPa　and　10.4%,　which　is　increased　by　7%　and　50%　than　Er-free　alloy,　respectively.