Abstract: The hardness of the 1Cr5Mo steel fastening nut of a supercritical unit steam turbine in a thermal power plant was reduced significantly after service. Aiming at this phenomenon, the aging tests (equivalent to service) of the 1Cr5Mo steel nut were carried out at 566 ℃. The effect of aging time on the microstructure and mechanical properties was studied, and the microstructure and performance were compared with those of the nut served at 566 ℃ for 10⁵ h. The results show that at temperatures not higher than 566 ℃, the microstructure of the 1Cr5Mo steel nut changed slowly; the microstructure was still martensite and carbide after aging for 2 880 h. But the microstructure changed to tempered sorbite after service for 10⁵ h. With the extension of the aging/service time, the carbides migrated and concentrated from the inside grain to the grain boundary, and became obviously coarsened. The deterioration of the structure led to the reduction of the hardness, strength, and impact energy of the nut.