<p>The transformation behavior in martensitic 9.5Cr-1.5MoCoVNbNB heat-resistant steel during heat treatment is systematically analyzed using scanning electron microscopy, transmission electron microscopy, and atom probe tomography. For all samples quenched at 1100&#xa0;°C and cooled by furnace-, air-, and oil-cooling, the martensitic lath contains M₃C. No fine precipitates were observed in the samples after quenched with water-cooling. Additionally, experimental results indicated that the steel exhibited a relatively high content of retained austenite in its microstructure after quenching followed by furnace-cooling. Tempering at 570&#xa0;°C maximizes the transformation of retained austenite into martensite and carbide. The matrix of steel tempered at 570&#xa0;°C for 4&#xa0;h contains M₃C, M₇C₃, M₂₃C₆, M₂₍C,N), and M(C,N). The precipitation of carbides in 9.5Cr-1.5MoCoVNbNB steel during tempering occurs in the following sequence: (1) M₃C → M₇C₃ → M₂₃C₆, (2) M₃C → M₂(C,N) → M₂₃C₆, (3) M₃C → M₂₃C₆. The precipitation transformation process is accelerated at higher temperatures and the steel tempered at 700&#xa0;°C contains M₂₃C₆ and M(C,N). Two-step tempering at 570&#xa0;°C and 700&#xa0;°C is adopted to improve the microstructure stability, facilitate the formation of retained austenite and metastable carbides, and ensure the dimensional stability of the 9.5Cr-1.5MoCoVNbNB heat-resistant steel rotor.</p>