The effects of Al5TiB and RE on microstructure and tensile properties of Mg-8Zn-4Al-0.3Mn casting magnesium alloys are studied.The results show that casting microstructures of Mg-8Zn-4Al-0.3Mn magnesium alloys mainly contain Mg matrix

eutectic α（Mg）

φ（Al2Mg5Zn2）phase and τ（Mg32（Al

Zn）49） phase.The morphology of ternary phases on the grain boundaries changes from quasi-continuous net to particle and the distribution of ternary phases gradually change depressively and homogeneously by addition of Al5TiB and RE.Casting microstructures of Mg-8Zn-4Al-0.3Mn magnesium alloys can be refined by addition of Al5TiB and RE

and the size of grains decreases from 120～130μm to 30～50μm.The tensile properties of the alloys at both ambient and elevated temperature are improved significantly by addition of Al5TiB and RE.