The effect of tool threads on the weldability and mechanical properties of 5-nm thick welded aluminum plates was investigated. The simplest shape, a circular cylinder tool without threads, and an ordinary shape, the same diameter tool with threads, were used to weld 1050-H24 and 6061-T6 aluminum alloys. The tensile strength of the joints using the tool without threads was slightly higher than or almost the same as that using the tool with threads. In the friction stir welded joints of the 1050-H24 and 6061-T6 aluminum alloys, the tool threads have very little effect on the weldability and mechanical properties. Defect-free joints had almost the same tensile strength and Vickers hardness profile for both tools. Temperature during welding and microtexture are also almost the same for both tools.