Abstract: The welded joints of dissimilar steels are widely used in ultra-supercritical power station boiler structure. The microstructure, elements and phase composition of T91/TP347H dissimilar welded joints in original state and service for 1×105 h were analyzed by OM, SEM, EDS and XRD, and the microhardness of these two welded joints was tested by microhardness tester. The results showed that, compared with the original state, grains of TP347H base metal after 1×105 h service were larger, the strengthening phases of T91 and TP347H in heat affected zone got coarsened, and M23C6 and Nb(C,N) precipitated in the weld. The maximum hardness of both welded joints appeared in the heat affected zone of T91, and the minimum hardness of both welded joints appeared in the base metal of TP347H. Compared with the original state, the hardness of welded joints after 1×105 h service increased significantly.Highlights: (1) The microstructure of welded joint of T91/TP347H dissimilar steel in its original state and 1×105 h of service were compared, failure causes of dissimilar joints were analyzed, which provided theoretical support for the safe application of T91/TP347 engineering.(2) The hardness variation trend of welded joint during service of dissimilar joint was systematically analyzed, and the precipitation variation rule of T91/TP347 dissimilar joint was clarified.