Abstract: The effects of heat treatment on the microstructure and mechanical properties of ZA84 (Mg-8Zn-4Al-0.25Mn) alloy were investigated. The results indicate that the as-cast microstructure of the alloy is mainly composed of α-Mg matrix and two different morphologies of precipitates (continuous and quasi-continuous Mg₃₂(Al,Zn)₄₉ phases and isolated Mg₅Al₂Zn₂ phases). After solid solution treatment at 345℃, the Mg₃₂(Al,Zn)₄₉ phases change from continuous and quasi-continuous net to disconnected acute angle shape, and parts of second phases sphericize. The optimum heat treatment condition for the alloy is solution treatment at 345℃ for 48 h and water quenching, then aging treatment at 200℃ for 12 h and atmosphere cooling. Under the optimum condition, the ultimate tensile strength and yield strength of the alloy can be imoroved, but the elongation is not effected much bv heat treatment.