The effects of trace Ca addition on microstructure and high-temperature creep resistance of as-cast Mg-12Zn-4Al-0.3Mn alloy were investigated. Results show that the as-cast microstructure of the base alloy consists of α-Mg matrix and block quasicrystalline phase at gain boundaries. The composition of this quasicrystalline phase is closed to that of the equilibrium t phase (Mg32(Al,Zn)49) in the Mg-Zn-Al ternary system. A small amount of Ca addition to the base alloy results in the formation of a lamellar eutectic j (Al2Mg5Zn2) and the transformation of the quasicrystalline phase to the equilibrium t phase in the as-cast microstructure of the alloy. The volume fraction of the j phase increases with the increase of Ca addition. The creep resistance of the alloys is significantly improved by Ca addition. The optimum creep resistance of the alloy is obtained by 0.6% Ca addition under the creep condition of 175 oC, 70 MPa, whose steady state creep rate is approximately one order of magnitude lower than that of its base alloy.