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Flexural and interlaminar shear properties of carbon fiber reinforced carbon (C/C) composite, fabricated by “the preformed yarn method”, were evaluated at elevated temperatures up to 2300°C in vacuum. Four point flexure test and short beam test were carried out on the C/C composites of unidirectional, cross-ply and quasi-isotropic laminates. Effect of absorbed gas on the composite strength was also studied. The results revealed that flexural strength increase with increasing temperature, and was not profoundly affected by absorbed gas. On the other hand, interlaminar shear strength (ILSS) was remarkably lowered by absorbed gas at room temperature, and the test on the degased specimen showed that ILSS was kept constant up to 2300°C. Hence the absorbed gas is shown to lower matrix strength but not fiber strength. Young's modulus of C/C composites was kept constant up to 1500°C, then decreased at 2000 and 2300°C, that might be originated form temperature dependence of the Young's modulus of the carbon fiber.