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°C, 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°C for 48 h and water quenching, then aging treatment at 200°C for 12 h and atmosphere cooling. Under the optimum condition, the ulti-mate tensile strength and yield strength of the alloy can be improved, but the elongation is not effected much by heat treatment. The results 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 a-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 ° C, 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 ° C for 48 h and water quenching, then aging treatment at 200 ° C for 12 h and atmosphere cooling. Under the optimum condition, the ulti-mate tensile strength and yield strength of the alloy can be improved, but the elongation is not effected much heat treatment.