Abstract: SiC_{1. 12}O_{0. 12} ceramic powders were obtained by pyrolyzing the high-branched polycarbosilane (HBPCS). The oxidation behavior of the powders in the dried air was investigated by the thermal gravimetric method. The mean particle size of the HBPCS derived SiCO ceramic powders was about 7μm. The samples were oxidized at temperatures of 900℃，950℃，1000℃ and 1050℃ for 4 h. The results show that the mass change during the oxidation process exhibits two distinguished stages. At the first stage (from 80 min at 900℃ to 20 min at 1050℃)，the mass is rapidly increased with the oxidation time. However，the mass of the powders changes little at the second stage due to the pore sealing. The molecular oxygen diffusion is the controlling process for oxidation at this stage. The activation energy of this material oxidized at 1173～1373 K is about 121 kJ/mol. The relationship between parabolic rate constants obtained by the thermal gravimetric method and oxide thickness change is deduced.