Abstract: The effects of 0%, 1%, 3% and 5% mish-metal additions on the microstructure and properties of an as-cast Mg-9Y-0.6Zr (WK90) alloy were investigated by optical microscope, X-ray diffractometer (XRD), scanning electron microscope (SEM), differential scanning calorimeter (DSC) and mechanical tester. The results indicate that the microstructure of the as-cast alloy is composed of α-Mg and little eutectic structure. After adding mish-metal, the eutectic structure increases markedly. When the mish-metal contents are 0%, 1% and 3%, the eutectic structure is in a single form, and there are the layered eutectic structure and the divorced eutectic structure when the mish-metal content is 5%. The areas of low temperature endothermic peaks in the DSC curves of WK90 alloy with different mish-metal contents increase and at last are separated with adding mish-metal, which is decided by the quantity and type of eutectic structure. The as-cast alloy with a mish-metal addition of 3% and the extruded alloy with a mish-metal addition of 1% have the best ultimate tensile strength under their own states. The influence reasons are not only grain size but also the distribution and the morphology of the divorced eutectic structure.