The authors have developed a new friction stir welding (FSW) tool that enables welding of high-softening-temperature materials, such as steels, and titanium alloys. The new tool is made of a Co-based alloy strengthened by precipitating intermetallics, Co3(Al,W), with an Ll2 structure at high temperatures. The Co-based alloy tool can be manufactured at a low cost through a simple production method consisting of casting, heat-treatment, and then machining. It exhibits yield strengths higher than 500 MPa at 1000°C, so that it might have great potential as a FSW tool for high-softening- temperature materials. In this study, the feasibility to use the Co-based alloy tool to friction stir weld various high-softening-temperature materials was examined. Changes in tool shape during FSW and the weld appearances produced with the Co-based alloy tool will be briefly shown.