The principal scheme of Friction Stir Welding (FSW) and the main technology process parameters that affect the formation of the join are presented. Different options for the formation of the contact interface «tool – welded sheets» are considered, depending on the type of the tool shoulder. The material flow and the joining mechanism of thin sheets of aluminum alloy with a thickness of 2.0 mm during the butt joint by the FSW method were performed. The process of the oxide layers transition from the surfaces and butts of the sheets to the joint is described and the formation of the remnant oxide layer in the stir zone of the joint during FSW is considered. It has been established that during FSW butt joint several processes of metal joining take place simultaneously. It is shown that the Friction Stir Welding method can be used in many industries to butt-weld thin and ultra-thin sheets with the choice of optimal technology process parameters and geometric characteristics of the tool.