The wet corrosion by high temperature-high pressure water has been studied on the three types of Si-Ti-C-O system (Tyrano) fibers at 300∼800°C under 100 MPa for 2h. The X-ray diffraction analysis and SEM observation of the scale layers and the measurement of the mass change of reaction products revealed that the pyrolysis together with forming β-SiC crystal took place in lower temperatures, while the hydro-thermal oxidation with following the formation of amorphous silica films propagated in higher temperatures. The oxidation reaction was sharply reduced as the amor-phous silica crystallizes to form cristobalite, whence the porous oxide layer is densified. The densification hindered the gaseous diffusion through the oxide film. The fiber containing 18% O (rich type), where atomic arrangement was disordered, showed a comparatively rapid pyrolytic rate. On the contrary, the fiber containing 12% O (poor type) showed a rapid oxidation rate.