Abstract: Friction stir butt joints of 6061-T4 aluminum alloy plates with a thickness of 12 mm were welded by various pin-tip profiles of the tool. The morphology of different cross section of key hole was observed, and the influence of pin-tip profile on joint-root material flow behavior was analyzed. The results show that the metal of FSW weld zone migration along the weld thickness direction is mainly affected by the thread of pin surface. The impact of the pin-tip taper on the joint-root metals migration along the weld thickness direction can be divided into two aspects, one effect is on downward migration of metal breakaway from pin thread, the another is on upward migration of weld nugget metal. The diameter of pin-tip affects the thickness of incomplete penetration of FSW butt joint. With the diameter of pin-tip face increases, the thickness of pin-tip affected zone increases, and thicher plate will be welded well.