The microstructure and bio-corrosion behavior of binary Mg-xZn (x ¼ 1.25, 2.5, 4) and ternary Mg-Ca-xZn (x ¼ 1.25, 2.5, 4) alloys have been studied using scanning electron microscopy (SEM), electrochemical, and immersion tests. Microstructure analysis indicated that the binary Mg-Zn alloys are composed of primary a-Mg matrix and Mg 12 Zn 13 phases, while, ternary Mg-Ca-Zn alloys are composed of a-Mg, Mg 2 Ca, and IM1 (Ca 3 Mg x Zn 15Àx ) (4.6 x 12) phases or a-Mg, IM1 and IM3 (Ca 2 Mg 5 Zn 13 ) phases. Electrochemical results showed that Mg-4Zn alloy has lowest corrosion rate among binary alloys. At constant Ca content of 0.8 wt.%, the addition of Zn up to 1.25 wt.% decreased the corrosion rate, while further addition of Zn increased the corrosion rate of ternary alloys. Immersion tests results demonstrated that the formation of Zn oxide layer in binary Mg-Zn alloy and evolution of eutectic phase (a-Mg þ IM1 þ Mg 2 Ca) significantly retard the bio-degradation rate of the ternary alloys.