Abstract: Ultrasonic-assisted friction stir welding (UA-FSW) was carried on 6061-T6 aluminum alloy plates with thickness of 3 mm, and the welding quality and mechanical properties of UA-FSW joint were studied, as well as tensile fracture and microstructure of weld nugget zone, and compared with those of FSW joint. The results show that UA-FSW could effectively reduced welding defects and widen welding process window. For the two joint, microhardness curves all exhibited “W” shape, and “onion ring” feature was found in weld nugget zone, but more smooth “onion ring” were obtained in UA-FSW joint. Comparing with FSW joint, UA-FSW joint owned higher microhardness and tensile properties, more and deeper dimples on tensile fracture, and smaller and more uniform grains in weld nugget zone.