The microstructures and mechanical properties of as-extruded and peak-aged Mg-6Zn-1Mn-2Sn-0.5Ca(ZMT612-0.5Ca) alloy were studied using optical microscopy (OM), X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), hardness tests and uniaxial tensile tests. The results showed that the as-cast alloy consisted of α-Mg, Mn, Mg7Zn3, Ca2Mg6Zn3 and CaMgSn phases. The as-extruded ZMT612-0.5Ca alloy presented a totally recrystallized microstructure. The average grain size of extruded alloy was ~2.8 μm. After heat treatment, the strength increased obviously, the yield strength and ultimate tensile strength of as-extruded alloy were increased to 320 and 390 MPa, respectively. Strength enhancement was attributed to the grain boundary strengthening, solid solution strengthening and precipitation strengthening.