Structural failure of a high pressure orifice flange of an oil refinery is studied by using visual inspection, optical and electron microscopy, tensile tests, fractography and stress calculation. This flange was located at a high pressure (2700 KPa), elevated temperature (370 C°) feeding pipe line. Failure observed during a startup at the time of increasing pressure and temperature. The results of the analysis show that brittle fracture has acted as a main factor for failure of the structure and stress concentration together with corrosion have provided minor sources for the failure. These results indicate that brittle behavior of this flange can attribute to hydrogen embrittlement of the steel material.