Abstract: Leakage reason of T23 steel tube in high temperature reheater was researched through a set of tests of macro check,tensile test,microstructure examination and theoretical calculation. The results show that the main reason for the leakage was due to oxide corrosion which resulted in the reduction of the tube’s wall thickness. Microstructure aging and mechanical deterioration of the tube caused by higher fuel gas temperature near the leaked tube accelerated the leakage.