Abstract: An H13 hot die steel punch fractured during extracting after drilling and forming of drill pipe. The reason for the fracture of the punch was studied by fracture morphology observation, chemical composition analysis, microstructure observation and micro-area composition analysis. The results show that the fracture type of the punch was early fatigue brittle failure. The locations of large inclusions of Al₂O₃, MnS, VC and defects such as pore and porosity in the outer wall area of the punch work site were easy to produce stress concentration, and became the fatigue crack source. Under cyclic thermal stress, tensile stress, compressive stress and bend load, the lateral microcracks intiated and extended rapidly in divergence form, which eventually led to the fracture of the punch. The content of Mo and V of the punch was far below the lower limit of standard values, resulting in decrease of high temperature mechanical properties of the punch, and then the fatigue fracture of the punch was accelerated.