The effects of the welding tool rotation speed on the microstructures and mechanical properties of friction stir welding joints of 4 mm thick 2205 duplex stainless steel sheet were studied. The results indicated that when the welding speed is 50 mm/min and the rotation speed is in the range of 600 ~ 800 r/min, the sound welds can be obtained. The grains in the stir zone of the joint were refined by the dynamic recry stallization, and the hardness value in this zone was relatively high. The grains in the heat affected zone were coarsened by the welding heat and the hardness value in this zone was relatively low. The ferrite content of the obtained joints was within the range of 50% − 60%, and the ferrite content in the stir zone was increased with the increase of rotation speed. When the rotational speed was 600 r/min, the tensile strength of the joint reached the maximum 824 MPa, which was 97.3% of the base material, and the fracture location was in the heat affected zone.