The oxidation behaviour and the effect of oxidation on the room-temperature flexural strength of B4C-30 wt % SiC composite material were investigated. The weight changes of the samples exposed to air at temperatures between 500 and 1000 degrees C were continuously monitored with a microbalance. At temperatures below 800 degrees C, the weight change of the specimen was negligible. As the temperature was increased to 800 degrees C, parabolic weight gain was observed; The rate of the weight gain increased with exposure temperature. The oxidation product formed on the surface was found to be a crystalline boric oxide (B2O3) by X-ray analyses. The oxide layer was severely cracked due to the thermal expansion mismatch between the oxide layer and the substrate. However, the room-temperature flexural strength was increased when the samples were exposed at temperature between 700 and 900 degrees C, apparently due to the blunting of strength-limiting defects at the surface. When the temperature was higher than 1000 degrees C, a severe reduction in strength was observed. The reliability of the composite material was also improved significantly by such exposures.