Abstract: Leakage occurred at the elbow of steel pipes of final superheater of supercritical unit boiler in a power plant. The leakage causes of steel pipes were analyzed by means of macroscopic examination, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis, and mechanical property test. The results show that the deformation of steel pipe elbows were too large in bending, and the ambient temperature of the outer arc side was too high, which resulted in stress concentration. With the deformation and aging of the steel pipes microstructure, there were chromium poor area near the grain boundary, and the strength of the grain boundary decreased. When the stress concentration reached the degree of grain boundary cracking, wedge-shaped cracks appeared at the junction of the grains. The rapid expansion of cracks eventually led to the leakage of the steel pipes during operation.