Tensile　test　of　the　as-cast　Mg-6Zn-2Er　alloy　was　conducted　at　room　temperature.　The　results　indicate　that　the　alloy　is　inclined　to　failure　when　the　strain　reaches　5.6%.　The　coarse　secondary　phases　are　responsible　for　the　failure,　especially　for　the　Mg3Zn3Er2　phase　(W-phase).　It　is　indicated　that　the　existence　of　the　W-phase　activates　the　stress　concentrations　due　to　the　incapacity　of　W-phase　for　the　load　transfer,　which　results　in　the　void　at　the　inner　of　the　W-phase.　In　comparison,　the　interface　between　the　matrix　and　the　secondary　phase　is　stable.　In　conclusion,　the　characters　of　the　secondary　phases　with　respect　to　size,　distribution,　morphology　and　type,　play　an　important　role　in　the　plastic　deformation　behavior　of　the　alloy.