A T300 carbon fiber and a SiC-coated C/SiC composite made from the same fiber were studied in atomic oxygen environment. The carbon fiber shows significant degradation while the erosion rate of SiC-coating of C/SiC is about 50 times lower. Evidence shows that Si is preferentially etched from the SiC surface. And XPS information showed that amorphous carbon and diamond-like carbonare periodically generated on the tested composite surface. Statistical analysis shows that the C/SiC specimens have no significant change in flexural properties after 1-year fluence AO treatment.