Abstract: Friction stir welding(FSW)is a rather new solid state joining process,usually for joining low melting temperature alloy.In this paper,the typical LF6 aluminum alloy was joined by using FSW process.The influence of the welding conditions on the weld appearance and mechanical properties were investigated.The reasons of forming defect in FSW have been analyzed.It is shown that the strength of the joint may be equal to that of the parent materials,the bending angle can reach 180°.The mechanical properties of the joint are related to the welding variables,and there is a region in which the properties is higher than 90% of the parent materials.In some welding conditions,some special appearance,such as groove on one side of the weld surface or channel in the weld or plasticised flow stream,may occur,the position is related with the rotation direction of the pin.