New magnesium alloys based on Mg-Zn-Al-Ca quaternary system (ZAC alloys) have been developed by IMRA America, Inc. The 200-hour creep extension of the new ZAC alloys at 150°C and 35MPa is approximately one order of magnitude less than that of conventional Mg-Al based die cast alloy AZ91D, and is also slightly lower than that of aluminum die casting alloy A380. The tensile and corrosion properties of the new alloys are slightly better than, or comparable to, those of AZ91D alloy. It is generally known that the die-castability of Ca-containing magnesium alloys is relatively poor due to the hot-cracking and die-sticking problems associated with the calcium additions. However, this problem has been resolved in ZAC alloys where the high zinc content (∼8%) can restore the die-castability. The present paper summarizes the alloy design, microstructure, die casting process, mechanical and corrosion properties of the new ZAC alloys. The bolt-load retention test results and the prototyping program on ZAC alloy for automotive power-train applications are also discussed in this paper.