To investigate the high-temperature steam oxidation behavior and creep life of T91 martensitic ferritic heat-resistant steel used in supercritical and ultra-supercritical (USC) power plants, the microstructure of the oxide layer on in-service T91 pipes was characterized using optical microscopy and scanning electron microscopy. After service, a bilayer oxide structure with the Fe-rich outer layer and the Cr-rich inner layer was formed in T91 steel. Creep voids and microcracks are found in the oxide layer. Moreover, MX, M23C6, and Laves phases have been observed in the matrix. A creep life prediction model for high-temperature heated pipes was established to calculate the oxide layer growth and creep life. The thickness of the predicted oxide layer is 128 μm after 20,000 h service, which exceeded the actual thickness. After 115,352 h service, the creep damage reached the critical damage value, indicating the failure of T91 pipe.