Abstract: In this work, the effect of Mn on the microstructure, mechanical properties, thermal conductivity and plastic deformation mechanisms of Mg-15.0Zn (wt%) alloys prepared by high-pressure die-casting was investigated. The results indicate that the Mn addition refined the grain size of the Mg-15.0Zn (wt%) alloy from 6.82 μm to 5.22 μm, and the eutectic phase slight coarsened, also improves the casting properties and eliminates cracks generated during the casting process. TEM diffraction analyses and quasi-in situ EBSD results showed that Mn addition activated prismatic slip and <c+a> slip during tensile deformation. Furthermore, mechanical tests showed that the addition of Mn significantly improved the ultimate tensile strength and of the alloys from 287 MPa for Mg-15.0Zn (wt%) alloy to 317 MPa for Mg-15.0Zn-0.3Mn (wt%) alloy. Meanwhile, the thermal conductivity increased from 68.9 to 78.6 W·K-1·m-1, owing to the addition of Mn which reduces the concentration of Zn atoms. Our work provides insights into the plastic deformation mechanisms of ultra-high Zn die-casting Mg alloys.