Objective To investigate the effect of nano-Y2O3 pretreatment on oxidation resistance of T91 steel in high-tempera-ture water vapor environment. The action mechanism of rare earth elements and the failure mechanism after long-term oxidation were also discussed. Methods The surface of T91 steel was coated by a slurry composed of Y2O3, aluminum powder and a binder. Then the coated sample was treated in water vapor environment at 720 ℃ for 48 h. Cyclic oxidation test of the pretreated samples and bare sample were conducted in water vapor environment at 700 ℃ . The effect of pretreatment on the oxidation resistance of the material in high-temperature water vapor environment was examined by comparison of cyclic oxidation kinetics between the pretreated sample and the bare sample. The composition, structure, morphology of the cyclically oxidized samples were investigated using XRD, SEM and EDS. Results Cr2 O3 -rich scale with small amount of Y2 O3 was formed on the surface of the pretreated sample. The pretreated samples showed slight weight gain after 600h oxidation in water vapor environment at 700 ℃ . The mass gain was about 0. 152 mg / cm2 at 600 h of oxidation. However, significant mass gain was observed when the oxidation time was longer than 600 hours. The pretreatment scale gradually degenerated during the oxiadation, and some Fe2 O3 grains were observed on the sample surface. For bare sample, significant mass gian was observed at the beginning of the oxidation stage. The mass gain was about 12. 1 mg / cm2 after oxidation for 70 h. Spallation of the oxide was observed during the oxidation, indicating poor adhesion of the scale to substrate. Conclusion Nano-Y2 O3 pretreatment could obviously improve the water vapor oxidation resistance of T91 steel. However, the Cr2 O3 -rich scale would degrade gradually during the oxidation in water vapor environment at high temperatures.