The 00Cr13Ni5Mo2 supermartensitic stainless steel was first tempered at 570-730°C for 2 h to observe the microstructure and hardness changes. The tempering temperature was set to 600, 650, and 700°C, which is below, equal to, and above the austenite transforma-tion start temperature, respectively, for each holding period to investigate the effects of tempering time on the structure and properties of the steel. The microstructure of the specimens was examined by optical microscopy and transmission electronic microscopy, and the phase composition was detected by X-ray diffraction. As expected, lath refinement was observed in the steel tempered at 700°C, and the refinement degree significantly depended on the tempering time. Contrary to normal steel softening by tempering, the hardness performance of the steel was significantly enhanced primarily because of the refinement of martensite laths after single-stage intercritical tempering. It is believed that the reverse transformation of martensite (α′) to austenite (γ) is responsible for the refinement.