New Mg-Zn-Al alloys, which have tensile strength and proof stress that are comparable to those of commercial magnesium alloys while exhibiting a wide solidification range and sufficient fluidity for semi-solid forming, have been developed. The effects of changing the quantity and ratio of aluminum and zinc contents and adding a small amount of strontium or calcium are also investigated. The microstructures of semi-solid formed specimens consist of uniformly distributed fine and globular solid particles surrounded by the liquid (eutectic) phase. The alloys with a small amount of strontium or calcium content show fine eutectic compounds. Some of the alloys completely fill a U-shaped mold cavity of 1 mm thickness and a total flow length of 120 mm. In addition, heat-treated samples having reduced zinc and aluminum content with a small addition of strontium or calcium exhibit remarkable increase in tensile properties. The presence of Mg-32(Al,Zn)(49) compound as well as the refinement of the eutectic compounds due to the addition of strontium or calcium are responsible for improving the mechanical properties of these alloys.