Low-oxygen silicon carbide fibers (Hi-Nicalon) were oxidized at temperatures from 1073 to 1773 K under an oxygen partial pressure of 0.25 atm. The strength of the unoxidized core was practically unaffected by the oxidation temperatures. The strength of the as-oxidized fibers with an SiO2 film decreased markedly with increasing oxidation temperature. When exposed subsequently to 1773 K in argon, the fibers with a SiO2 film of 0.3-0.5 mu m thickness had the best thermal stability.