Abstract: Single-pass and double-pass friction stir processing（FSP） tests were performed on 6 mm thick AZ31B magnesium alloy plate. The microstructure and mechanical properties of the stir zones were investigated by optical microscopy, electron backscattered diffraction, Vickers hardness, and tensile tests. The results show that the grain size in the stir zones is significantly refined due to continuous and discontinuous dynamic recrystallization. Compared to single-pass FSP,the average grain size of double-pass FSP slightly increases, the ratio of low angle grain boundaries increases, and the Schmidt factor value significantly reduces. Grain boundary strengthening is the dominant factor for improving the strength of the stir zone. Due to the lower Schmidt factor, the basal slip is difficult to be activated; the interaction between twins and dislocations produced during the stretching process significantly improves the elongation after fracture, therefore, the double-pass FSP AZ31B magnesium alloy exhibits a relatively good match of strength and ductility.