Abstract: Crack was found on welded joint of a T91 steel steam sampling tube in power plant ultra-supercritical unit boiler after using about 4 200 h. The cracking causes were analyzed through macroscopic inspection, microstructure analysis, tensile test, fracture analysis and micro-hardness testing. The results show that the martensitic structure of the heat affected zone near base metal was obviously decomposed into sorbite, and the coarse Laves phase and M23C6 phase distributed on the grain boundaries continuously. These changes in microstructure not only resulted in the decrease of strength and hardness of the area, but also brought about grain boundary weakening. Under the combined action of high temperature and multiaxial stress, the welded joint eventually generated type IV crack failure.