Abstract: Aiming at performance failure of non-oxide materials under high temperature water vapor conditions, the reaction behavior of hexagonal BN powder with the average particle size of 1.2 μm at a temperature range of 1273 K to 1373 K in water vapor (the volume ratio of H₂0/air was 3:7) was investigated by thermogravimetric analysis, X-ray diffraction and scanning electron microscopy, together with a comparison with the reaction behavior of BN powder in dry air. The reaction process of BN powder in aqueous conditions has the following characteristics:the weight gain rate increases quickly in the initial stage of reaction, but it becomes slow at the later stage of reaction. The reaction mechanism was discussed together with thermodynamic calculation. It is shown that the weight gaining is attributed to oxidation of BN powder to form B₂O₃. A reaction of B₂O₃ with H₂O forming volatile products causes the weight gaining rate become slowly. In both stages, the weight gain rate all increases when the temperature rises. The reaction kinetics of BN powder in high-temperature water vapor is fitted precisely and quantitatively by Chou's model, and the results get a good agreement with experimental data.