A study has been made on the tensile and creep properties of squeeze and die cast Mg alloys, with emphasis on the effect of second phase particles. Two different microstructures were compared; the microstructure with thermally unstable particles along grain boundaries (AZ91 alloy) and the microstructure with thermally stable particles along grain boundaries as well as within matrix (MX alloy). The experimental MX alloy has equivalent room temperature tensile strength to AZ91 alloy with higher ductility. Moreover, elevated temperature tensile properties and creep resistance of MX alloy are higher than those of AZ91 alloy. Such improved properties of MX alloy over those of AZ91 alloy are due to the optimized microstructure of the former.