The effects of heat treatment on microstructures and hardening response of Mg–6Zn–0.5Er–0.5Ca(wt%) alloy were investigated by optical microscope(OM), scanning electron microscope(SEM), and transmission electron microscope(TEM) in this paper. The results show that the Mg–6Zn–0.5Er–0.5Ca alloy contains Mg_3Zn_6Er_1 quasicrystalline phase(Iphase) and Ca_2Mg_6Zn_3 phase under as-cast condition. Most of the Ca_2Mg_6Zn_3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn_2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn_2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which increases about 40 % compared with that of as-cast alloy. The effects of heat treatment on microstructures and hardening response of Mg-6Zn-0.5Er-0.5Ca (wt%) alloys were investigated by optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope The results show that the Mg-6Zn-0.5Er-0.5Ca alloy contains Mg_3Zn_6Er_1 quasicrystalline phase (Iphase) and Ca_2Mg_6Zn_3 phase under as-cast condition. Most of the Ca_2Mg_6Zn_3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ° C for 36 h, a large amount of MgZn 2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn_2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which increases about 40% compared with that of as-cast alloy.