Isothermal oxidation of silicon hexaboride (SiB6) ceramic powder at high temperature was investigated. The SiB6 powder samples were heated from room temperature to 1273K for 25 hours in air, and weight changes were measured to estimate the oxidation resistance and nature of the oxide layers on the SiB6 ceramics. The samples oxidized In the temperature range of 673 to 1273 K for 25 hours exhibited increasing weight gain with increasing oxidation temperature. The oxidation proceeded in accordance with the parabolic law during the initial oxidation stage. The weight gain increased with increasing oxidation temperature. Based on the results of,X-ray diffraction analysis, silicon oxide (SiO2) and boron oxide (B2O3) were present on the surface of samples oxidized in the temperature range of 873 to 1273 K.