Abstract: As the main reason for the vertical flow of the plastic material during FSW, pin thread geometry plays an important role on material flow and mechanical properties of friction stir overlap welded(FSLW) joints. In order to investigate the effect of the half-threaded pin on the microstructure and mechanical properties of FSLW process, the full-threaded pin and the half threaded pin were used to welded alclad 2024 aluminum alloy, respectively. Cross-section of the joint, lap shear failure load, fracture position were studied. Results show that the half-threaded pin easily causes the hook to bend downwards, leads to the bigger effective sheet thickness and lap width. The fracture mode is shear fracture, and FSLW joint with the half-threaded pin owns the higher shear failure load.