Abstract: Friction stir welding (FSW) lap joints of 3 mm-thick LY12 aluminum alloy sheets were made using a cylindrical tool with right-hand threaded pin,and the effects of plunge depth of shoulder on the interface migration and mechanical properties of resultant joints were investigated. The results show that the lap interfaces on both sides of weld nugget moved downward, and the height of interface migration rose with increase of plunge depth of shoulder. The height of interface migration was greater on the retreating side than on the advancing side. The interfaces on both sides of weld moved slightly when using an appropriate plunge depth of shoulder. The shear strength of lap joint was excellent due to the slight interface migration,and fracture occurred in the top sheet. However,when the interfaces at both sides of weld moved downward,fracture occurred in the bottom sheet. The shear strength decreased with increase of the height of interface migration. The shear strength was higher when the load was exerted on the advancing side than on the retreating side.