Abstract: The effect of the tool tilt angle on the characteristics of LF5 aluminum alloy by friction stir welding such as the material flow and mechanical property was introduced. Tunnel defect is observed at 0.5 mm below the tool shoulder as the tilt angle θ is less than 1.5°. Material at lower portion of nugget is deposited. With the increase of θ, deformation and material flow of nugget become more serious, the nugget become wider and the defect disappears gradually.When θ is 2°, kissing bond was found in the upper region through the observation of fracture by SEM. If θ is between 3.5° and 4.5°, the nugget gains its largest width, the joint obtains the max tensile strength (350 MPa) and toughness(16%).