The oxidation behavior and thermal stability of Si-C-O fibers (Nicalon) in CO2 gas were examined at 1273 to 1773 K through mass change determination, XRD analysis, resistivity measurement, SEM observation and tensile test. Large mass gains and SiO2 film formation show that Nicalon fibers are passively oxidized in CO2, as well as Si-C fibers (Hi-Nicalon). The grain growth of beta-SiC, the drop of resistivity and the crystallization of SiO2 film were observed above 1573 K. On being oxidized at high temperature, the thermal decomposition of SiCXOY and the formation of imperfections in SiO2 film caused the degradation of the room-temperature tensile strength of both the as-oxidized fibers and unoxidized cores. It became evident that the oxygen partial pressure for the active-to-passive oxidation transition of Nicalon fibers is between 1 00 x 10(2) and 1.78 x 10(2) Pa at 1773 K.