The reaction behavior of α-Boron nitride (BN) powder in wet air was investigated at 1273 K using thermogravimetric (TG) analysis, X-ray diffraction (XRD), and mass spectrometry. Scanning electron microscopy and electron probe X-ray microanalysis (EPMA) were used to analyze the morphological and elemental development of the sample. The results show that the reaction process consists of two stages: the oxidation of BN powder to form B2O3 and the further reaction of B2O3 with H2O. In the first stage, the oxidation reaction occurs very quickly and can be described by Chou's model. In the second stage, the reaction rate followed the linear rate law.