A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carried out to investigate the mechanical properties and microstructural evolution of 20Cr32Ni1 Nb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides(Nb C and M23C6). In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of Nb C and M23C6 carbides during service condition, the Ni-Nb silicides known as G-phase(Ni16Nb6Si7) are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32Ni1 Nb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time–temperature–transformation curve for the steel is obtained from the aged specimens. A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carried out to investigate the mechanical properties and microstructural evolution of 20Cr32Ni1 Nb steel. Experimental results indicate that the as-cast microstructure of the steel the most consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides (Nb C and M23C6). In addition to the growth and coarsening of Nb C The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitation of M23C6 carbides during service conditions, the Ni-Nb silicides known as G-phase (Ni16Nb6Si7) are formed at the interdendritic boundaries. rate of G-phase, depending on part on Si content, varies greatly for the 20Cr32Ni1 Nb steel, which plays a ke based on the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time-temperature-transformation curve for the steel is obtained from the aged specimens.