The microstructures of oxide scales formed on MOSi(2) at medium-high temperatures in air were observed by TEM. Based on the observation, relationships between oxidation temperature and formation of MoO(3) and crystallization of amorphous SiO(2) scales were investigated. At 1273 K and 1373 K, the oxide scales had a structure consisting of amorphous SiO(2) with small amounts of fine MoO(3) particles. The oxide scales at 1573 K and 1773 K had a structure consisting of amorphous and crystalline SiO(2). Growth rate of the oxide scale formed at 1773 K appreciably increased due to crystallization of amorphous SiO(2). It was thought that the increase in the oxidation rate at 1773 K was caused by a change in the diffusion mechanism from O(2) diffusion to lattice diffusion of O(2-) through SiO(2). In addition, the diffusion coefficient of oxygen was estimated from the growth rate Of SiO(2) scale.