First-principles shear and tensile tests were performed on pure Mg, Mg-Zn-Ca, Mg-Ca, and Mg-Zn models, and the generalized stacking fault energy for basal and prismatic slip systems and the surface energy of the {0001} plane were investigated to understand the origins of the high-stretch formability of Mg-Zn-Ca alloy. The calculations suggested that the high formability of Mg-Zn-Ca alloys is attributable to improved plastic anisotropy, not to the enhanced surface energy.