Abstract: Cracking and leakage of the extraction steam drain elbow of a steam turbine in a nuclear power plant occurred after four years of service,and an axial crack was found on both sides of the elbow.Field investigation found 4 girth welds and repair welding at two ends of the elbow.The leakage reasons were analyzed by means of macro examination,chemical compositions analysis,metallographic examination and SEM and EDS analysis.The results show that because there was a certain content of chloridion in the pipe medium,stress corrosion cracking and corrosion fatigue cracking happened under the coactions of welding residual stress and thermal cycling stress,which resulted in the leakage of the elbow.