The effect of CO treatments on thermal stability of low-oxygen SiC fibers (Hi-Nicalon) was examined at 1273 - 1773 K using mass change measurements, X-ray diffraction (XRD) analysis, Auger electron spectroscopy (AES) analysis, resistivity measurements, scanning electron microscopy (SEM) observation and tensile tests. The fiber properties remained unchanged by heating below 1573 K. In addition to the grain growth of SiC, reduction of resistivity and degradation of strength above 1573 K, mass loss was observed above 1673 K. AES analysis showed carbon film formation on fiber surfaces at high temperature. The carbon film was formed by the following reaction:CO(g) + SiC(s) = SiO(g) + 2C(s)A Tensile strength of 1.83 GPa was retained even after exposure at 1773 K for 10 h, owing to the suppressing effect of the carbon film on the thermal decomposition of SiCXOY phase. (C) 2003 Kluwer Academic Publishers.