Abstract: 12Cr1MoVG/TP304H dissimilar steel tube welded joint of final boiler reheater in a power plant fractured during service, and the fracture cause was analyzed by macroscopic morphology observation, chemical composition analysis, mechanical property test, microstructure observation, micro-area composition analysis. The results show that the brittle fracture occurred in dissimilar steel tube welded joints along the fusion line on 12Cr1MoVG steel tube side. After long-term service at high temperature, the carbon in the welded joint diffused and migrated, and then the carbide concentrated near the fusion line on 12Cr1MoVG steel tube side distributed along the grain boundaries; meanwhile, micro creep cracks formed, which seriously weakened the strength and toughness of the region. The thermal conductivity and the coefficient of linear expansion between 12Cr1MoVG steel and TP304H steel was very different, resulting in the formation of large thermal stress in the high temperature environment. Finally, the dissimilar steel welded joint underwent brittle fracture along the fusion line on 12Cr1MoVG steel tube side under the combination of high temperature steam pressure and thermal stress.