Oxidation behavior of silicon nitride (Si3N4) was investigated in flowing air (2.45 cm/s) containing 10%-50% H2O at a total pressure of 1.8-10 atm at 1300°-1500° C for 100 h. The oxidation Of Si3N4 progressed with volatilization of the SiO2 scale; it was more enhanced at a high partial pressure of H2O rather than at high temperature. The total pressure had little effect on the oxidation. In order to avoid the oxidation, Si3N4 substrate was coated with lutetium disilicate (LU2Si2O7) layer through the intermediate SiO2-rich phase. While the coating layer well suppressed the oxidation in case of small amount of water vapor, it was not sufficiently effective to suppress the oxidation when the water vapor was rich. SiO2 volatilization was observed between the layer and substrate. The flexural strength of the coated Si3N4 at room temperature was somewhat increased after the oxidation in wet air, while that of the uncoated one was almost unchanged. This increase was attributable to crack healing of the substrate during the oxidation.