Abstract: A drivepipe flange of 42CrMo for oil and gas extraction fractured during service. The fracture reasons were investigated by macroscopic observation, fracture analysis, composition and inclusion examination, microstructure and surface quality analysis. It was found that the 42CrMo flange had excessive brittle inclusions and poor surface quality at the tooth root, leading to the fracture. Accordingly, such measures as improving the control of metallurgical and finishing processes were adopted, and these measures were found to be effective.