During corner AdStir fillet stationary shoulder friction stir welding (FSW), filler material with surface oxide is fed into the stir zone. In this study, the material flow of the filler material and surface oxide layer during the process was investigated by microstructure observation. To visualize the material flow, a marker insert technique was employed. Similar to conventional FSW, fine and equiaxed grain structure was observed in the stir zone. EBSD investigation revealed that the material flow was governed by the simple shear deformation induced by the rotating probe. The filler material was widely distributed in the stir zone, suggesting that some amount of the filler material moved downward due to the vertical material flow. The initial surface layer on the filler material was finely broken up by the material flow, achieving metallic bonding between the filler wire and the plates. Any harmful effects by adding the filler material were not found in the mechanical tests in this study.