25Cr3Mo3NiNb steel is an outstanding material for special pressure vessels, in which thermal fatigue cracking is a critical factor limiting its service life. The strength of this steel is primarily derived from secondary hardening induced by dispersed carbides, whose size strongly depends on the diffusion rate of key alloying elements at different temperatures. Herein, the effect of thermal fatigue temperature on the crack propagation behavior of 25Cr3Mo3NiNb steels, being subjected to quenching and tempering treatments respectively was studied. The thermal fatigue test was conducted via helding the steel at 600 ^oC or 700 ^oC for 5 min and then quick cooling in room temperature water as one cycle. The variation of microstructures and crack propagation behavior along with the thermal cyclic testing up to 300 cycles was characterized using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The results show that after thermal fatigue at 600 ^oC, the M₂C carbides coarsened slightly while still maintaining a high number density, which effectively retarded the microstructure recovery and preserved the tempered martensite structure, accompanied by a slight decrease in hardness. In contrast, after thermal fatigue at 700 ^oC, the M₂C carbides coarsened significantly, resulting in the transformation of the microstructure into tempered sorbite and a remarkable drop in hardness. The difference in microstructural state directly governed the crack propagation mode: at 600 ^oC, both the main and secondary cracks propagated primarily along pre-austenite grain boundaries where severe dislocation pile-ups occurred; at 700 ^oC, microstructural recovery eliminated local inhomogeneity, leading to transgranular propagation of the main crack, while the secondary cracks tended to nucleate and extend along inclusions.