Abstract: 3D-C/SiC composites, exposed in air at 600, 900, and 1300℃ for 0 to 15h, were investigated by three point bend tests at room temperature, SEM, and energy dispersive spectroscopy. The results show that the damage curves, expressed as a relative change of elastic modulus, of the composites for a 15h exposure, could be divided into a sharply increasing stage (stage I) and a steady increasing stage (stage II). Stage I may be caused by a direct oxidation of the carbon fibers and interface carbon layers by the oxygen in air, and stage II may be caused by a diffuse controlled oxidation of the inner part of the composites. The matrix micro-cracks, induced by a difference of coefficients of thermal expansion between matrix and carbon fibers in the cooling process after composite preparation act as oxygen diffuse paths and are where the oxidation takes place. The fact that the damage decreases with temperature for the same exposure time may be caused by the crack shrinking at high temperature, which decreases the oxidizable surface area and inhibits the diffusion of oxygen into the composites.