The authors have developed a new friction stir welding (FSW) tool that allows the welding of high-softening-temperature materials (HST.Ms) , such as steels, titanium, and titanium and zirconium alloys. The new tool is made of a cobalt (Co)-based heat-resistant alloy strengthened by precipitated intermetallics,Co3(Al, W), with a A2 structure. The Co-based alloy tool exhibits yield strengths higher than 500 MPa at 1000 deg C, so that it has great potential as a tool material for FSW of HSTMs. In this study, the feasibility of using the Co-based alloy tool with titanium and titanium alloys was examined. Changes in the tool shape during FSW and the weld characterization produced with the Co-based alloy tool will be briefly shown.