Directionally solidified Manesium alloys AZ91 with element Y were produced by using the liquid metal cooling Bridgman furnance. The dendrite morphology and microstructure of AZ91 were investigated in present paper. The results indicated that the primary dendrites of AZ91 grow along the direction of heat flow. The are six secondary dendrites which around the primary dendrite likes petal shape can be observed in cross section. The microstructure of AZ91 alloy is mainly composed of a matrix of primary a(Mg) solid solution and eutectic b(Mg17Al12) phase, and some Y-rich phase can be found with Y additions by using SEM and EDS. The RE-rich phases present coarse rod-like shape or tiny graininess. The Y, Mg, Al and Mn elements are found in RE-rich phase, and the mass fraction of rare earth element Y is about 22%.