Al-4.5Zn-1.0Mg-0.8Cu　alloys　containing　different　RE　content　were　prepared　by　metal　mould　casting.　Effects　of　RE　on　microstructure　and　mechanical　properties　of　Al-4.5Zn-1.0Mg-0.8Cu　alloy　were　investigated　by　tensile　testing,　OM(optical　microscovpe),　SEM(scanning　electron　microscope)　and　EDS(energy　dispersive　spectrometer).　The　results　show　that　the　tensile　strength　and　elongation　are　increased　firstly,　and　then　decreased　with　the　increase　of　RE　content.　The　alloy　with　0.12%　RE　(mass　fraction)　exhibits　the　desirable　mechanical　properties.　With　proper　RE,　secondary　dendrite　arm　spacing　can　be　decreased,　and　RE　is　distributes　mainly　at　grain　boundaries　and　dendrite　boundaries　in　form　of　AlZnMgCuRE　phase.　Excessive　RE　can　cause　over-modification　effect,　increasing　secondary　dendrite　arm　spacing.　With　a　lower　RE　content,　AlZnMgCuRE　phase　exhibits　rod-like　shape,　and　with　excessive　RE,　AlZnMgCuRE　phase　becomes　coarse　block　structure,　which　can　weaken　the　mechanical　properties　as　a　result　of　cutting　effects.　In　addition,　RE　can　promote　the　solid　solution　of　Mg　and　Zn　in　Al　melt,　improving　the　solid　solution　strengthening　effects.