Recently, several metallic materials with ultrafine-grained structures and characterized by high strength and toughness have been developed. When these ultrafine-grained materials are practically used, welding and joining processes are required. However, conventional fusion welding processes result in deterioration of the good mechanical properties of these ultrafine-grained materials due to the drastic grain growth of the ultrafine grains. On the other hand, friction stir welding (FSW) is a solid-state joining process having lower heat-input than fusion welding processes, enabling formation of a fine grain structure in the stir zone. Thus, this process would effectively alleviate deterioration of mechanical properties of the ultrafine-grained materials. The authors applied FSW to ultrafine-grained Al alloys and then examined the microstructural features associated with hardness in the friction stir welds. The present paper reviews microstructural evolution of ultrafine-grained Al alloys, produced by equal channel angular pressing (ECAP) and accumulative roll-bonding (ARB), during FSW.