Abstract: The high temperature superheater tubes of a 300 MW boiler in a power plant cracked during hydraulic test. By means of macro observation, chemical composition analysis, metallographic examination, hardness test, tensile test, micro analysis, X-ray energy spectrum analysis and so on, the cracking causes of high temperature superheater tubes were analyzed. The results show that there were obvious decarbonization layer and corrosion pits on the inner wall of the high temperature superheater tubes. The decarbonization layer reduced the corrosion resistance of the tubes, and the residual stress existed in the elbow and the weld. The combined action of the two resulted in stress corrosion of the tubes until cracking.