The effects of RE on the microstrcture of as-cast Mg-8Zn-4Al magnesiu m alloys were investigated. The results show that the solidification range of Mg-8Zn-4Al -xRE alloys increases with RE additions. A binary eutectic reaction can arise and produce a new phase (Mg3Al4Zn2RE) and the temperature of phase transformation point of the ne w phase is 412.85℃. In Mg-8Zn-4Al-1.5RE alloy, a small amount of Mg3Al4Zn2RE phase and ε phases are found besides α(Mg),φ and τ phases. Also microstructures of Mg-8Zn-4Al alloy s can be refined by addition of 1.5% RE obviously. The results of RE on the microstructure of as-cast Mg-8Zn-4Al magnesiu m alloys were investigated. The results show that the solidification range of Mg-8Zn-4Al-xRE alloys increases with RE additions. A binary eutectic reaction can arise and produce a new phase (Mg3Al4Zn2RE) and the temperature of phase transformation point of the new phase is 412.85 ℃. In Mg-8Zn-4Al-1.5RE alloy, a small amount of Mg3Al4Zn 2RE phase and ε phases are found besides α (Mg), φ and τ phases. Also microstructures of Mg-8Zn-4Al alloy can be refined by addition of 1.5% RE obviously.