Grain refinement is an established technique which basically reduces the grain size of material with the direct intention to improve 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 chemical grain refiner. AZ91 is a cast magnesium alloy used to produce die cast component while the basic microstructure of it alloy consists of primary α- phase in which aluminum rich β- phase (Mg17 Al12) is precipitated along grain boundaries is a cast magnesium alloy. As cast AZ91 alloy generally exhibits low strength and ductility due to the network like eutectic β- Mg17 Al12 destributed at the grain boundaries.In this present review article, the research methods grain refinement and effect that on characterization of microstructure and mechanical properties of mgAZ91 cast alloy.