Abstract: The fatigue fracture characteristics of diffusion pipes for Z6CNT18-10 austenitic stainless steel in steam hydrophobic environment were studied by means of metallographic microscope, scanning electron microscope and X-ray diffraction. The results show that the fatigue cracks of diffuser pipes for austenitic stainless steel in steam hydrophobic environment originated from the inner wall, and they were unlike common fatigue cracks which often originated from the outer wall. There was obvious oxidation phenomenon at the crack source. There were some stationary slip bands in the microstructures near the cracks.The fatigue bands had very narrow spacing. There was almost no the final fracture zone in the microstructures, which belongs to high cycle fatigue.