Abstract: Oxidation kinetics of siliconmonocrystal have been studied. The three steps rule of gas-solid reaction have been raised. The reaction rate of oxidation is divided into three stages:in the initial period the reaction rate is controlled by interface chemical reaction; in the second stage by both interface chemical reaction and slow diffusion due to stress; in the third stage by diffusion.And the microstructure of oxide film have been observed by SEM and TEM.