AZ31B magnesium alloy was joined by friction stir welding (FSW) and submerged friction stir welding (SFSW), respectively, and the microstructure and mechanical properties of the joints were investigated. The experimental results show that the nugget zone in SFSW joint is characterized by fine and equiaxed recrystallized grains. Increasing of the travel speed will induce an increase in grain size and a decrease in microhardness in the nugget zone. The microstructure of the FSW joint on the surface is coarser and more inhomogeneous than that in the center, while the microstructure of the SFSW joint on the surface is obviously finer than that in the center. The microhardness of the FSW joint on the surface is lower than that in the center, while the SFSW joint has the opposite trend. When the rotation speed is 950 r·min-1 and the travel speed is 75 mm·min-1, the maximum ultimate tensile strength of the SFSW joint reaches 72% that of the base material. The fracture surface of the SFSW joints exhibits cleavage-like feature