Abstract: A friction stir processing tool is formed from a Ni-based dual multi-phase intermetallic compound alloy containing rhenium(Re). The Ni-based dual multi-phase intermetallic compound alloy is preferably formed by casting, while gradually cooling, a melt containing all the components of the composition and is preferably heat treated after casting. Moreover, in a method for friction stir processing, a work is softened by friction heat generated when the friction stir processing tool, while rotating, is pressed against the work to be processed. The friction stir processing tool includes the Ni-based dual multi-phase intermetallic compound alloy, and therefore further high hardness is exhibited to improve abrasion resistance, so that even a long period of friction stir processing can be endured.