Abstract: A through-wall crack occurred in a domestic A105 flange used in gas pipeline during the gas pressure testing at -16℃.The chemical composition and mechanical properties as well as formation and propagation of crack were analyzed by means of optical microscopy,scanning electron microscopy and material property tester.The results show that the elongation and impact toughness of the failed flange were decreased as a result of coarse grained structure and formation of Widmanstatten microstructure in local region.The machining line with no smooth bottom between the cone surface and tube surface in flange structure,which could cause stress concentration,became a source of crack.The crack propagated along brittle microstructure under the pipe pressure and welding residual stress which caused the flange cracking failure ultimately.