Glass/yttrium silicates/SiC multi-layer coating for carbon/carbon (C/C) composite was prepared and the flexural strength and weight changes of the coated samples after oxidation in air and combustion atmosphere at 1500oC were investigated. Results show that when oxidation in air and combustion atmosphere, the C/C composites lost weight by a linear and a parabolic law respectively. The weight loss reaches its maximum value at about 800oC in air but the strength of the specimen arrives at the lowest value at about 850oC. There are three oxidation zones along the specimens. A non-oxidation zone is below 400oC. Close to it is the cracking-oxidation zone (400-1100oC). At the temperature range from 1100oC to 1500oC is the coating-oxidation zone. Fracture of the coated sample at the temperature spot of 850oC is observed after oxidation in combustion atmosphere at 1500oC for 70 hours, which are considered to be responsible for the cracking and decreasing in strength during the test in combustion atmosphere.