Abstract: Self-reacting friction stir welding was conducted on 3 mm thick ZK60 magnesium alloy at different welding speeds (100-400 mm·min^-1), and then the microstructure of the joint was studied. The results show that the weld nugget zone of the joint had a dumbbell-shaped cross section. Flow patterns were observed near upper surface of the weld nugget zone on retreating side at the 1/4 radius of the shoulder. The advancing-side thermal mechanical affected zone was comprised of a banded structure stretching to the weld nugget zone. At different welding speeds, the interface between the advancing-side thermal mechanical affected zone and the weld nugget zone was obvious. The materials in the advancing-side thermal mechanical affected zone were deformed plastically, and the dynamic recrystallization occurred partially. The advancing-side weld nugget zone had a dynamic recrystallization at different welding speeds, and consisted of fine equiaxed dynamic recrystallization grains at 300 mm·min^-1 and 400 mm·min^-1; the flow pattern in one direction disappeared.