Abstract: The microstructure and mechanical properties of AZ31B magnesium alloy rolled plate were studied upon annealing at different temperatures and for different times.Recrystallization behaviors of the alloys were investigated through microstructures observation and mechanical properties test.The results show that recrystallization took place during annealing of rolled plate.The tensile strengths of annealed alloy descended lightly,while the tensile elongation increased.The refine grain of alloy obtained with an average size about 10μm by annealing at 523K for 60min and the tensile strength and elongation are 258.12 MPa and 22.25% respectively which leads the general properties of the alloy further increase.Fracture behavior turns quasi-cleavage fracture into ductile fracture after annealing.