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In this paper, the microstructural evolution, grain refinement and mechanical properties of as-cast Mg-6Zn-0.4Zr alloys with varying Sm contents (0, 2 wt.%, 4 wt.% and 6 wt.%) were investigated by using an optical microscope (OM), a scanning elec-tron microscope (SEM) equipped with energy dispersive spectroscope (EDS), an X-ray diffractometer (XRD) and mechanical tests at room temperature, respectively. The experimental results indicated that the addition of Sm could obviously refine the as-cast grains, modify the eutectic morphology and affect the mechanical properties of the alloys. The main phases in Mg-6Zn-xSm-0.4Zr alloys in-cluded matrixα-Mg, Mg2Zn3, Mg41Sm5 and MgZnSm. With Sm content increasing to 4%, the MgZnSm phase was created, mean-while, the morphology of some eutectic phases revealed apparently lamellar structure, which had a bad effect on the mechanical prop-erties. In addition, the maximum values of ultimate tensile strength (UTS, 214 MPa) and elongation (EL, 7.42%) were simultaneously obtained from the alloy with 2% Sm. However, Sm additionhad no obvious effects on the fracture behavior of the alloys, namely, the fracture pattern of Mg-6Zn-0.4Zr alloy belonged to inter-granular and brittle modes while the fracture regimes of all the Sm-containing alloys weredominated by the mixture of inter-granular and trans-granular modes.