Grain refinement is an established technique that reduces the grain size of material with the direct intention of improving its finished properties or characteristic. The main objective of grain refinement is to produce a fine, uniform, and equiaxed grain structure. On the other hand, grain refinement of cast magnesium can be achieved by two methods represented by increasing the cooling rate and adding a chemical grain refiner. AZ91 is a cast magnesium alloy used to produce die-cast components. In contrast, the basic microstructure of this alloy consists of a primary αphase in which the aluminum-rich βphase (Mg17 Al12) is precipitated along grain boundaries in cast magnesium alloy. Cast AZ91 alloy generally exhibits low strength and ductility due to the network-like eutectic β-Mg17 Al12 distributed at the grain boundaries. In this review article, the research methods of grain refinement and its effect on the characterization of microstructure and mechanical properties of AZ91 magnesium cast alloy.