An Al-Mg-Sc-Zr alloy rolled sheet was subjected to friction stir welding (FSW) with a tool rotation speed of 500 r/min and a tool travel speed of 100 mm/min. The microstructure of the alloy sheet after FSW was characterized by OM, EBSD and TEM. The microhardness and strength were also tested. The results show that after the FSW, the fiber grains adjacent to the rotating tool are bent. The texture of the rolled sheet is gradually alleviated. In the nugget zone, the size of the tiny equiaxed grains is only 2.31 μm. Al₃(Sc,Zr) particles are also affected by the FSW. The distribution and the size of the particles after the FSW are different from those of the original sheet. Moreover, the shape of the Al₃(Sc,Zr) particles turns from spherical to polygonal and many particles become incoherent with the Al matrix. The tensile strength of the FSW sheet is 367 MPa and the yield strength is 263 MPa, and those of the original sheet are 414 and 311 MPa.