Polycarbosilane-derived SIC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10(2) to 10(5) Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 x 10(2) Pa and actively oxidized under an oxygen partial pressure of 10(2) Pa. Under oxygen partial pressures front 2.5 x 10(2) to 10(3) Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer, Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was last under the active-oxidation condition.