A new Ca–Mg–Zn alloys having significantly improved glass-forming ability (GFA) has been developed. The ternary Ca65Mg15Zn20 bulk metallic glass with diameter of at least 15 mm is successfully fabricated by conventional copper-mold casting method in air atmosphere. The critical cooling rate for glass formation in the cone-shaped copper mold is less than 20 K/s. When compared with the GFA of Ca–Mg–M (M: Cu, Ni, and Ag) alloys, the significantly improved GFA of the Ca–Mg–Zn alloy cannot be represented by ΔTx, Trg, K, and γ parameters.