The nitridation of silicon and oxidized‐silicon has been studied. The nitrided films were prepared at 900°–1150°C under ammonia partial pressures of 10−3 to 5 kg/cm2 in nitrogen and were analyzed by ellipsometry and Auger electron spectroscopy. For films formed by nitridation of silicon, we found that the growth kinetics and properties such as chemical composition, etching rate, and oxidation resistance were independent of the ammonia partial pressure. The nitridation of silicon can be explained by a modified Ritchie‐Hunt theory, which assumes that a very slow surface reaction at the ammonia‐nitride interface is the rate‐determining factor, using the logarithmic rate law. According to this modified Ritchie‐Hunt theory, the nitridation of silicon proceeds mainly by cation migration under a constant electric field. On the other hand, it was found that the nitridation of oxidized‐silicon depended strongly on the ammonia parital pressure. This dependence may be caused by diffusion of ammonia or its derivatives through the oxide. The conversion of silicon dioxide to silicon oxynitride occurred throughout the oxide.