Microstructure, age hardening response and mechanical properties of Mg-4.0Sm-xCa (x=0.5, 1.0 and 1.5 wt%) alloys during extrusion and subsequent isothermal aging at 200 oC were investigated. The results indicate that with the addition of Ca, the bulk and particle-like Mg41Sm5 phases containing Ca and the needle/rod-like Mg2Ca phases form in the Mg matrix, grains of the alloy are refined and tensile mechanical properties are improved remarkably. In T5 (peak-aging) condition, the Mg-4.0Sm-1.0Ca alloy shows the smallest grain size of 5.1 μm. The amount of Mg2Ca phase increases gradually, but that of the bulk Ca-containing Mg41Sm5 phase in alloy with 1.5 wt% Ca significantly decrease, which mainly distribute at the grain boundaries. The peak-aged Mg-4.0Sm -1.0Ca alloy exhibits the highest peak hardness (82 HV) and the optimal ultimate tensile strength, yield tensile strength and elongation of 267 MPa, 189 MPa and 24%, respectively. The improved mechanical properties of the alloy are attributed to grain refinement, solution strengthening and precipitation strengthening of Mg2Ca phase and Mg41Sm5 phase.