The reaction behavior of chemically vapor-deposited silicon nitride (alpha-Si3N4) powder in wet air at 1673-1873 K for 10 h has been investigated. The oxidation and volatilization reaction coexist initially while volatilization reaction progressively becomes the limiting step with extended reaction time. Based on the experimental curves, a new kinetic model regarding both solid product layer and hypothetically volatile product layer has been developed to interpret its short-term reaction and predict long-term corrosion behavior. The calculated results agree well with the experimental data. Predictably, this new kinetic model can not only be used to treat the reaction of Si3N4 powder in wet air but also can be applied to deal with the gas-solid reaction of other ceramic powders.