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. Compositions of the alloys were controlled to have four different microstructures; 1) having thermally unstable particles along grain boundaries, 2) having thermally stable particles along grain boundaries, 3) having thermally unstable particles along grain boundaries and thermally stable particles within matrix, and 4) having thermally stable particles along grain boundaries as well as within matrix. It shows that room temperature strength of the alloys is mainly controlled by the volume fraction of second phase particles, regardless of their distribution. The distribution of second phase particles, on the other hand, has a large effect on the fracture behavior. It shows that the creep resistance of Mg alloys can be significantly improved by the incorporation of thermally stable second phase particles within matrix. Such improved properties of TAS831-X alloy over those of AZ91 alloy are due to the optimized microstructure of the former.