Abstract: The effect of oxidation on internal friction behavior of C/SiC composites was investigated to lay foundation for evaluating the oxidation behavior using the internal friction, by analyzing the change of the porosity, mass, flexural strength, phase, microstructure and also measuring the internal friction by thermal mechanical analyzer during oxidation at elevated temperature (1250, 1300 and 1350℃) in air. The relationship between oxidation and internal friction for SiC ceramics was also studied at 1300℃ in air, to clarify the affect of oxidation of each component in C/SiC on the internal friction behavior. The results show that the effect of oxidation on internal friction for SiC is irregular and weak; the internal friction behavior of C/SiC is controlled obviously by the oxidation of carbon phase and the peak value of internal friction retention at 1250, 1300 and 1350℃ is 6.65, 3.48 and 1.59, respectively.