The specimens were cut from different extrusion direction, and the effect of annealing heat treatment on the microstructure and mechanical properties of AZ31B sheet were investigated. The results show that the sheet are composed of α-Mg primary phase and Al2Mg phase. The surface texture, which is formed in the course of extrusion deformation and parallelled to the plate （0001）, results the mechanical propertis of the sheet in different direction are different, and the mechannical propertis of the specimens in 90° direction are highest with compared to other directional specimens, the tensile strength is 274.3 MPa, and the elongation is 14.5%. The mechanical propertis of the annealing heat treatment specimens are increased, which are attributable to both the reduced grain size and the recrystallization microstructure, and the mechannical propertis of the specimens in 0° direction are increased apparently. In addition, the fracture characterics of the specimen is mixture of fragility and toughness in extrusion state, but it is changed to toughness fracture after annealing heat treatment.