The purpose of this study was to investigate semi-liquid forming process as it was applied to magnesium alloys and to characterize the effects of processing variables on forming process. Magnesium billet was produced by a grain refining method. The possibility of the semi-liquid method for producing the slurry with thixotropic property was considered. This was followed by heating process for the desired fine globular microstructure and the process for subsquently shaping the semi-liquid slurry. The mold filling behavior was investigated at various processing variables including injection velocity, injection pressure, and material temperature(liquid fraction of Mg alloy). The tensile property of the specimen was measured at the constant semi-liquid forming condition(injection velocity : 0.5m/s, injection pressure : 135㎏/㎠, liquid fraction : 0.6). Liquation initiated from the solute segregation parts and the shape of solid particles turned into spheres without stirring. It was observed that the mold filling behavior was improved as the injection velocity and the injection pressure increased. Unexpectedly, the tensile property of the specimen was measured a little lower than that of diecasting.