Abstract: Cracking failure occurred to the herringbone gear ring in a high speed gearbox during the second test. The cracking reason of the herringbone gear ring was investigated by macro-examination, chemical composition analysis, hardness test, metallographic examination and SEM analysis.The results show that the chemical composition, macrostructure, non-metallic inclusions of the gear ring are normal, and the main reasons of the cracking are the large stress concentration effect caused by microcracks on the rough machining surface of threaded hole, high thermal stress occurring in heat treatment, and the carburizing in thread. The large impact stress produced in starting promoted the propagation of the cracks.