A novel semi-solid processing technique, called new vacuum suction casting (NVSC), is used to manufacture high-quality components of AZ91D Mg alloy directly from a liquid metal. The resulting apparent morphologies and microstructures of castings are characterized in detail and linked to the corresponding mold-filling behavior and subsequent solidification behavior. It is revealed that the semi-solid metal (SSM) with higher viscosity can be caused to fill the mold with "solid-front fill", as compared with the liquid metal "spraying" in the conventional vacuum suction casting (CVSC) process. The smooth filling achieved in the NVSC process diminishes some disadvantages inherent for the CVSC sheets, and generates castings with better surface finish and structures with high integrity. The microstructure of the CVSC sheet consists of the fine and homogeneous supersaturated alpha-Mg solid solution due to the extremely high cooling rate. In the NVSC microstructure, the "preexisting" primary solid particles, with the morphology of near-globules or rosettes, disperse in the homogeneous matrix consisting of fine near-equiaxed secondary alpha-Mg grains and fine precipitates of beta-Mg17Al12 intermetallics. In addition, owing to rapid solidification, the volume fraction of the beta phase in the sheets obtained by both the processes is much lower than that in the as-cast ingot.