Abstract: Abstract: The thermal decomposition of pre-oxidized polycarbosilane (PCS) fibers was carried out in a thermogravimetric analyzer under non-isothermal condition. The Coats-Redfern equation was used to estimate the apparent activation energy in thermal decomposition and the mechanism of thermal decomposition was studied by IR, XRD and SEM. It was found that the thermal decomposition of pre-oxidized PCS fibers was a first order reaction and the apparent activation energy was estimated to be 19.826kJ/mol. The thermal decomposition process was a dehydrogenation and dehydrocarbonation condensation with the evaporation of low molecular components at low temperature. With increasing temperature the fibers were converted into an inorganic structure by decomposition of the side chains of the polymer. When the temperature was further increased, the crystallite size of SiC increased and the fibers became completely inorganic; above 1250℃. The mechanical properties of the SiC fibers became lower with the rapidly increasing crystallite size of the β-SiC.