We　have　systemically　studied　the　effect　of　the　erbium　on　the　microstructure　and　the　mechanical　properties　in　the　5xxx　series　aluminum　alloys　by　using　optical　microscope,　transmission　electron　microscope　(TEM)　and　by　tensile　testing.　The　results　demonstrate　that　the　tensile　strength　increased　quickly　at　the　beginning　of　small　contents　of　0.1　%Er　both　in　the　hot　and　cold　rotted　states,　then　slowly　increased　with　increasing　the　contents　of　Er　until　0.4%,　at　which　the　best　balance　of　the　strength　and　ductility　(438MPa　and　9.6%)　were　obtained.　Microstructure　observation　in　the　hot　rolled　state　was　indicated　that　the　grain　structure　in　the　Er　free　Al-5Mg　alloy　revealed　fully　recrystallized　grain　structure,　while　in　the　Al-5Mg　containing　Er　was　demonstrated　deformation　structure,　indicating　the　Er　addition　delayed　the　recrystallization　behavior　by　the　formation　of　the　precipitation　of　the　Al3Er,　which　confirmed　by　means　of　the　X-ray　diffraction　analysis.　Furthermore　in　the　TEM　microstructure　observation　the　precipitation　of　Al3Er　was　distributed　both　in　the　grain　interior　and　subgrain　or　grain　boundaries,　which　could　be　pinning　the　subgrain　or　grain　boundary　migration　and　dislocation　movement　as　well.　Consequently　the　beginning　of　the　recrystallization　temperature　in　the　Al-5Mg　containing　Er　was　elevated　about　50　degrees　C　than　in　Al-5Mg　without　Er.　This　could　be　explained　that　the　strength　increased　without　the　deterioration　of　the　ductility　was　attributed　to　the　microstructure　refinement　by　the　Er　addition.