3mm thick TC4 titanium alloy and 2A14-T4 aluminum alloy were well joined by friction stir welding (FSW), and the influence of tool offset on weld formation and tensile property of the joints was investigated. The results show that tool offset to the Al side has significant influence on ultimate tensile strength (UTS) of the joints. With the increase of tool offset, the UTS increases gradually. When the tool offset is 2.0mm, the UTS of the joint decreases with increasing tool rotation speed from 400r/min to 700r/min. However, when the tool offset is increased to 2.5mm, the UTS of the joint increase with the increase of tool rotation speed. A tool rotation speed of 700r/min and a welding speed of 60mm/min is matched, the highest UST of 347MPa of the joint is obtained, which reaches to about 83% of Al matrix. The tensile results prove that the fracture location and tensile strength of the joints mainly depend on microstructure and intermetallic phases. For the joint with the highest strength, the fracture occurs in the heat affected zone (HAZ) of Al matrix due to brittle TiAl phase formed in the joint.