Abstract: Cracks were found on a 16Mn steel tee in one sour gas field after a year of service,and the cracking reasons were analyzed by means of macroscopic observation,chemical compositions analysis,hardness test,metallographic examination,SEM observation and corrosion products analysis.The results show that the cracking of the tee was sulfide stress corrosion cracking,and the higher hardness and existence of upper bainite structure were main reasons for the cracking.It was recommended that the retesting for raw materials and production quality had to be strictly controlled,and non destructive testing should be performed periodically to the key positions in order to prevent similar incidents from happening again,including flaw detection,hardness testing,etc.