The　effects　of　different　contents　of　rare　earth　element,　and　erbium,　on　the　as-cast　microstructures　of　Al-6Zn-2Mg　and　Al-6Zn-2Mg-1.8Cu　alloys　were　studied　by　optical　microscopy,　scanning　electron　microscopy,　X-ray　diffractometry,　transmission　electron　microscopy　and　EDS　analysis.　The　results　show　that　the　netlike　structure　of　as-cast　alloys　can　be　remarkably　refined,　and　the　distance　of　dendritic　structure　decreases,　with　Er　addition.　However,　the　improvement　results　on　Al-Zn-Mg-Cu　are　not　better　than　that　of　Al-Zn-Mg.　Er　and　Al　can　interact　to　form　Al3Er　phase,　which　is　coherent　with　α(Al)　matrix,　with　trace　Er　addition　to　the　Al-Zn-Mg　alloy.　The　refinement　effect　of　Al-Zn-Mg　alloys　is　familiar　with　the　formation　and　precipitation　of　coherent　Al3Er　phases.　The　ternary　compound　AlCuEr,　similar　with　AlCuSc　phase,　will　form　when　Er　is　added　to　Al-Zn-Mg-Cu　alloy,　which　suppresses　the　formation　of　Al3Er　phase　and　doesn＇t　solve　in　the　following　heat　treatment.