The　results　of　X-ray　diffraction　(XRD),　differential　scanning　calorimetry　(DSC)　and　microstructure　observations　have　shown　that　various　Zn/Er　weight　ratio　leads　to　different　phase　compositions　in　the　investigated　alloys.　The　composition　range　for　the　formation　of　icosahedral　quasicrystalline　(I-phase)　and　face-centered　cubic　phase　(W-phase)　in　Mg-rich　corner　is　obtained.　The　Zn/Er　weight　ratio　adjusting　from　10　to　6　results　in　the　formation　of　I-phase　exclusively　as　main　secondary　phase　in　the　as-cast　Mg-Zn-Er　alloys.　However,　when　the　ratio　is　less　than　0.8,　it　is　mainly　contributed　to　the　precipitation　of　W-phase.　When　the　ratio　is　in　the　range　from　1　to　4,　I-phase　coexisting　with　W-phase　is　inclined　to　be　formed.　The　results　of　tensile　tests　suggest　that　the　as-cast　Mg-5Zn-5.0Er　alloy　with　I-phase　and　W-phase　possesses　the　highest　ultimate　strength　(UTS).　It　is　concluded　that　UTS　and　elongation　rate　(ER)　of　as-cast　Mg-Zn-Er　alloys　with　I-phase　as　the　main　secondary　phase　are　excellent.　The　UTS　of　the　alloys　with　coexistence　of　I-phase　and　W-phase　show　the　tendency　of　increasing　firstly　then　decreasing　with　the　increasing　volume　fraction　of　W-phase.　The　UTS　declines　when　W-phase　completely　replaces　I-phase　in　the　alloy.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.