Abstract: A membrane blower coupling fractured during the trial operation. Means such as macro examination, metallographic examination, chemical compositions analysis, and mechanical properties test were used to analyze fracture reason of the coupling. The results show that overheated microstructure caused by unsuitable heat treatment was the main reason for the frcature, which resulted in poor toughness of the coupling. And the stress was not released in time by annealing. Finally early brittle fracture of the coupling occurred in service. After quenching and tempering treatment to the fractured coupling, the impact toughness was greatly improved, and the comprehensive mechanical properties of the coupling were excellent.