Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO2 gas. The oxidation of the fibers was Investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of beta -SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO2 film were observed for the fibers after heat treatment in CO2 gas. SiO2 film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO2 gas (p(O2) = 1.22 Pa at 1273 K - 1.78 x 10(2) Pa at 1773 K), Hi-Nicalon Fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO2 film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.