The　microstructure　and　mechanical　properties　of　the　Mg-6Zn-xEr　alloys　in　the　as-cast,　as-extruded　and　extruded-T5　states　were　investigated.　It　is　found　that　the　addition　of　Er　has　an　obvious　effect　on　improving　mechanical　properties　of　Mg-6Zn　based　alloys.　The　results　suggest　that　the　Mg-6Zn-0.5Er　alloy　in　the　peak　aged　state　shows　the　best　tensile　strength.　The　ultimate　tensile　strength　and　the　yield　tensile　strength　of　the　peak-aged　Mg-6Zn-0.5Er　alloy　are　about　329　MPa　and　183　MPa,　respectively,　companying　with　a　good　elongation　of　12%.　The　addition　of　0.5%　Er　into　the　Mg-6Zn　based　alloys　has　a　great　effect　on　improving　ageing　hardening　response.　The　highest　tensile　strength　of　the　peak-aged　Mg-6Zn-0.5Er　alloy　is　mainly　due　to　the　refinement　of　microstructure　and　the　precipitation　of　beta(1)　phase.