Friction stir welding (FSW) using polycrystalline cubic boron nitride (PCBN) tool was tried on a 430 ferritic stainless steel. The microstructural evolution and mechanical properties in the weld were examined. The base material (BM). having coarse elongated grain structure, changed to fine duplex microstructure of martensite and ferrite matrix in the stir zone (SZ) and the thermomechanically affected zone (TMAZ) during FSW. It is suggested that martensite transformation in the regions was promoted by both rapid cooling rate and high strain induced by severe plastic deformation caused by frictional stirring. The hardness of the SZ was much higher than that of the BM, which was explained by the martensite formation and finely distributed duplex microstructure. The 0.2% proof stress and ultimate tensile strength of the SZ were roughly twice higher than those of the BM. However, the SZ showed half the elongation of the BM due to large amounts of martensite.