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ZrB2–SiC composites have been developed for aerospace applications related to hypersonic flight owing to their unique properties of high strength, modulus, hardness, moderate toughness and oxidation resistance. For the high temperature anti–ablation structural applications, the fracture strength of the ZrB2-SiC composites on high temperature oxidation is a critical factor. However, the quantitative relationships between the fracture strength of ZrB2–SiC composites on high temperature oxidation and temperature and various parameters of microstructures are barely known. In this work, based on the traditional fracture theory and the fracture thought at high temperatures, a theoretical method for studying the temperature dependent fracture strength and critical flaw size of the ZrB2–SiC composites on high temperature oxidation is proposed. The combined effects of temperature, microstructure and oxidation damage are taken into account by this method. The method can be easily used to predict fracture strength of the composites and its key control mechanism by using some basic material parameters. The predictions are in good agreement with experimental reports. The theoretical method can become a potential convenient and practical technical means for determining temperature and oxidation dependent fracture strength and critical flaw of the ZrB2-SiC composites developed for aerospace applications related to hypersonic flight. The research is useful for materials scientists and engineer community for the design of ceramic composites for high temperature application.