Thermal stability of low-oxygen SiC fiber (Hi-Nicalon) coated with SiO2 film was investigated. The SiO2 film of same thickness but different crystal structure was formed by heating low-oxygen SiC fiber (Hi-Nicalon) under different oxidation conditions. The oxidation treatment and the subsequent exposure at 1773 K in argon caused very little loss of strength for unoxidized core. For as-oxidized fiber with SiO2 film which contained imperfections, further loss of strength was caused after exposure in argon. There was little degradation of core strength on being exposed repeatedly at rapid heating and cooling rate in argon. The fiber oxidized at 1773 K kept high level of strength even after exposure at 1823 K. This is because the change in crystal structure of SiO2 film before and after exposure in argon, which was the controlling factor in the degradation of strength, was diminished with increasing oxidation temperature.