Abstract: The hydraulic tube-expanding mandrel of tube hole and tube sheet for steam generator in a company fractured repeatedly during the hydraulic pressure expanding process, and fracture reason of the mandrel was studied by instruments of optical microscope, scanning electronic microscope, tensile testing machine and direct reading spectrometer. The results show that stress concentration easily formed at the concave position of thread of the mandrel because fixed tooling was fixed only on one side, which led to low cycle fatigue fracture of the mandrel and shortening its service life. Some suggestion for hydraulic tube-expanding process was proposed finally.