Abstract: Microstructure and tensile properties at room temperature and elevated temperature of main steam piping of 12Cr1MoV steel after long-term (26 a) service were studied, and compared with those of the unused steel pipe. The results show that the structural degradation of the long-term served steel pipe was characterized by obvious spheroidization of pearlite, obvious fragmentation of layered Fe₃C phase, precipitation of M₇C₃ carbides in ferrite and significant growth of grains; Compared with those of the unused steel pipe, the yield strength at room temperature of the long-term served steel pipe decreased significantly, the tensile strength at room temperature changed little, and the yield strength and tensile strength at elevated temperature decreased greatly. The tensile fracture at room temperature of the long-term served steel pipe had typical fiber zone and tear zone, indicating a ductile fracture mode; while during tensile at elevated temperature, quasi-cleavage fracture occured.