Tube leakage is one of the most frequent causes of Once Through Steam Generator (OTSG) tube failure. A leaking tube was discovered within the OTSG unit of the Cogeneration Plant of 100 MW. The failed section of the tube was removed from the steam generator. Several metallurgical examinations of this tube segment were performed to identify the failure mode and cause. The results of the metallurgical evaluation revealed that the tubing was furnished to the chemical requirements of ASME Specification SB 407 and UNS N08800 alloy. The subject tube failure involved a through-wall crack that occurred as stress corrosion cracking. Residual stresses in the tube, created from forming of the tube at the time of manufacturing, combined with flow-induced stresses experienced in service as the steam generator operated, resulted in the formation of longitudinal cracking. The cracking formed and extended gradually into and through the tube wall, influenced by corrosion products in the cracks until the failure occurred. Additions of solutions used in boiler water treatment practices potentially induced corrosive substances into the tube. It is suggested that the tubes, especially the lower ones, of the OTSG are not located in a confined space.