PNC-FMS is a newly developed 11Cr-ferritic/martensitic steel with good swelling resistance designed for the wrapper tubes of fast reactors. Because fusion welding of PNC-FMS significantly reduces mechanical properties through the formation of a brittle microstructure, friction stir welding (FSW) was attempted as a solid state welding process. FSW was applied to PNC-FMS at 100 to 300 rpm using a Q60 tool, and defect-free welds were obtained. The stir zones had a fine microstructure with ferrite and martensite with the grain size and fraction of martensite increasing with tool rotational speed. Since all welds were overmatched, all transverse tensile specimens failed in the base material. Based on mechanical properties in the stir zone, welds produced at 100 rpm exhibited both higher strengths and elongation than the base material. This study showed that FSW at lower rotational speed produced stir zone microstructures having better mechanical properties in PNC-FMS.