Abstract: In order to study the effects of stir pin shapes on microstructure and tensile properties of friction stir welded joint,friction stir welding tests were carried on 2A14 aluminum alloy with 8 mm thickness by stir tools with different pin shapes,and the effects of the shapes of stir pin on microstructure and tensile properties of the joint were studied.The results show that,increasing the "trimming" depth,on the one hand,could enhance the stirring action and promote the flow of the plastic material,and obtain a good welded joint,on the other hand,could enhance the ratio of dynamic volume to static volume,increase the extrusion action of stir spin on material and heat production of material deformation,increase the area of the nugget zone and thermo-mechanically affected zone.The "onion ring" in nugget zone is the performance of sufficient flow of material.The ratio of dynamic volume and static volume being over 1 is very important to the improvement of the tensile properties of the joint,but after the ratio up to 1.10,further increasing the ratio has no significant effect on the tensile strength,but still has evident effect on the elongation after fracture.