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 a-Mg matrix and two different morphologies of precipitates (continuous and quasi-continuous Mg32(Al,Zn)49 phases and isolated MgsAl2Zn2 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 improved,but the elongation is not effected much by heat treatment.