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Abstract: The effects of antimony (Sb) on the microstructures of as-cast Mg–8Zn–4Al (ZA84) magnesium alloy were investigated by using optical microscopy, SEM, EDX, and XRD, etc. The results indicated that the microstructures of as-cast alloys containing Sb consisted of solid solution α(Mg), ternary phases τ [Mg32(Al, Zn)49] and φ (Al2Mg5Zn2), eutectic mixture phase [α(Mg)+τ], a newly formed dual phase MgZn2, and a newly formed needle-like phase Mg3Sb2. After adding Sb, the morphologies of ternary phase on the grain boundaries have been changed from quasi-continuous-net shape to uniformly dispersed particles, and the sizes of grains were decreased from 120–130μm to 50–60μm. In this experiment conditions, the optimal addition amount of Sb was 0.3wt%. Meanwhile, the microhardness of the matrix of ZA84 magnesium alloy was found to increase with the increases of Sb additions. As the phase Mg3Sb2 with high melting point and the dual phase MgZn2 were formed, the high-temperature strength of ZA84 magnesium alloy was increased. [Copyright &y& Elsevier]