Abstract: The AZ31B magnesium alloy sheets were welded by friction stir welding and the microstructure, phase composition, hardness and corrosion resistance of the welded joints were investigated. The results show that the weld nugget zone of the joint was composed of small equiaxed grains and the grains in the thermal-mechanical affected zone were refined and elongated along metal plastic flow direction. The microstructure of the heat affected zone was close to that of the base metal but the grain size was slightly larger. Comparing to the base metal of the welded joint, the phase composition of the weld seam had no change but the hardness decreased slightly. The hardness in different zones first decreased then increased with the increase of the distance from the weld seam center and the lowerest hardness appeared in the thermal-mechanical affected zone. The corrosion resistance of the joint was slightly lower than that of the base metal. The surface of the joint was general corroded after immersion corrosion. However, the corrosion degrees of the different zones of the joint were different. The corrosion of the thermal-mechanical affected zone was the most serious.