The thermal oxidation of a silicon nitride film was evaluated at temperatures ranging from 900°C to 1100°C and under water vapor partial pressures ranging from 0.25 atm to 0.95 atm. The oxidation rate was measured by ellipsometry and the results were compared with those of simultaneously oxidized silicon substrates. The conversion ratio, which is the thickness ratio of a grown oxide film to a consumed nitride film, of 1.64 was obtained with small fluctuation. The oxidized nitride thickness xn was found to be proportional to t2/3, where t is the oxidation time. This relationship is consistent with a proposed oxidation model. The figure of merit m was defined as the masking effect of a nitride film. It was found that a large m value was obtained when the film was oxidized at low temperature and under high water vapor partial pressure.