Injection molding for magnesium alloys is a new process in which metallic slurry is injected into a mold at semi-solid temperature to form net-shape products from solid feed stock in one step. The relationship between the process parameter and mechanical properties of AZ91D magnesium alloy has been investigated. It is shown that injection molding could produce parts with superior mechanical properties to those of high pressure die casting. High injection velocity refines the grain size of the surface and improves the mechanical properties.