Change in mass and gas concentration during oxidation of carbon matrix composite added with B4C and SiC particles as an inhibiter for oxidation were measured continuously and accurately using a thermogravimetric analyzer, a couple of zirconia type oxygen sensors and infrared CO-CO2 gas analyzer. Total amount of oxygen consumed, the amount of oxygen consumed for matrix carbon oxidation, the amount of oxygen incorporated in CO and CO2 gases, total oxygen consumption rate during oxidation of the composite were obtained. The amount of glass phase formed from the inhibiter and the amount of matrix carbon oxidized could be estimated from calculation of a mole balance between the total oxygen consumption and the amount of oxygen incorporated in CO and CO2 gases. Oxidation behavior and oxidation mechanism of the C-B4C-SiC composites were clarified.