Effects of tool geometry on mechanical properties of spot friction welds in lap-shear specimens of 6061-T6 aluminum sheets are investigated based on experimental observations. Three tools with shoulder diameters of 8 mm (small), 10 mm (medium) and 12 mm (large) were used to join aluminum sheets, respectively. The experimental results indicate that the failure strengths of the welds show significant dependence on the tool size. Optical and scanning electron micrographs of the welds made by three tools before and after failure were examined and micro indentation tests of these welds were conducted. The results show that the weld geometry, microstructure distributions and micro hardness distributions of the three welds are quite similar. However, the material flow trends of these welds appear to be quite different. The small and medium failed welds show the circumferential/shear failure mode and the large failed weld shows the nugget pullout failure mode.