Mg-5.8Sm-0.4Zn-0.3Zr（SZ58K） alloy was prepared by metal mold gravity casting

and the alloy prepared was treated by solid solution

and aging curve was drawn.The microstructure and mechanic properties of Mg-5.8Sm-0.4Zn-0.3Zr alloy in different heat treatment states including solid solution and ageing treatment were investigated by optical microscope（OM）

scanning electron microscope（SEM） and X-ray diffraction analysis（XRD）.The results show that the as-cast alloy consists of α-Mg matrix and Mg41Sm5 phase near the grain boundaries.After solid solution treatment

the second phase is decomposed

and small amount of square-like phase（mainly Sm） can be found in the grain with slight growth of the grain size.The change of yield strength can be ignored

while the ultimate tensile strength and elongation are remarkably increased.After aging treatment

yield strength is increased greatly

and ultimate tensile strength is increased slightly

while elongation is sharply decreased.