Semi-solid processing is an emerging technology for near net-shape production of engineering components. Although there has been significant progress in semi-solid processing of Al alloys, very limited information is available on semi-solid processing of Mg alloys, except for the thixomolding process. In semi-solid casting process, it is necessary to properly control the flow and solidification behavior of semi-solid slurries for high quality products. There are a number of parameters that affect the flow and solidification behavior of semi-solid slurries such as viscosity, casting pressure, shapes of gate and mold cavity, gate velocity, mold temperature, etc. In the present study, the effects of various thermo-mechanical treatments were investigated on the change in viscosity of the semi-solid AZ91D magnesium alloys by using a concentric cylinder type viscometer. The effects of gate velocity and thickness on mold filling behavior of the semi-solid AZ91D alloys were also investigated by using a high-speed camera and the results were compared with those obtained from computer simulations. From these results and microstructure examination, a processing map for high pressure die casting of the semi-solid AZ91D alloy was constructed in order to produce sound castings.