The welding of thick plates and high-temperature strength materials with friction stir weldingFriction stir welding (FSW) becomes difficult due to the insufficient rigidity of the welding machine. Multilayer build-up FSW enables welding of thick plates or hard materials with low-stiffness machines. In the present study, the feasibility of two-layer multi-pass FSW was examined through tensile test and microstructureMicrostructure analysis of the 10-mm thick butt joints of 6061 aluminum alloyAluminum alloys plates, which have rectangular opened grooves. The first layer consists of one-pass without supplying build-up metal. After reforming the groove shape by millingMilling, the first-pass of the second layer is conducted supplying rectangular rods as the building-up metal. The opened grooves are successfully filled by build-up metal. The width of the stir zone of a multi-layer build-up FSW joint differs from that of an ordinary FSW joint, since the plunging depth of the probe per pass is less. The multi-layer build-up FSW joints cause tensile fracture inside the stir zone. It was found that the broadening of the heat-affected zone of the joint is suppressed.