Al-5Mg　alloys　containing　different　content　of　Er　were　prepared　by　cast　metallurgy.　The　effect　of　trace　Er　on　the　microstructure　and　properties　of　Al-5Mg　alloy　was　studied　with　tensile　properties　measurement,　OM,　XRD,　SEM,　TEM　and　EDS.　The　results　show　that　the　addition　of　Er　to　Al-5Mg　alloy　can　improve　the　strength　remarkably,　and　the　yield　strength　of　the　cold-rolled　alloy　is　improved　by　81　MPa　when　the　content　of　Er　is　0.4%　(mass　fraction),　while　the　ductility　keeps　stable　or　decreases　slightly.　The　addition　of　Er　can　reduce　the　dendritical　arm　space　of　the　alloy,　and　the　grains　are　refined　obviously.　The　refinement　mechanism　of　Er　addition　on　the　grain　depends　on　the　quantity　of　Er　added.　The　refinement　mechanism　of　the　alloy　containing　less　Er　conforms　to　be　traditional　refining　theory　on　rare-earth　aluminum　alloy.　The　grain　of　the　alloy　containing　more　Er　is　refined　due　to　the　formation　of　primary　Al3Er,　and　the　primary　Al3Er　shall　be　able　to　act　as　nucleus.　The　strengthening　effect　of　Er　on　Al-5Mg　alloy　is　caused　by　grain　refinement　and　the　fine　secondary　Al3Er　particles.　The　results　indicate　that　the　alloys　with　0.4%Er　have　more　excellent　properties.