Abstract: The reducing tee pipe fitting used in a petrochemical plant leaked during the hydrostatic test. The leakage causes of the reducing tee pipe fitting were analyzed by means of macro observation, chemical composition analysis, metallographic examination, fracture and energy spectrum analysis, mechanical property test. The results show that there were casting inclusions (slag) and folding defects in the process of pipe making. In the process of cold extrusion, cracks occurred along the original defects at the transition diameter part with the largest deformation, so leakage occurred in the subsequent hydrostatic test.