Abstract: An approach to predict the elastic modulus of unidirectional C/SiC composites at 400~900 ℃ oxidation environments was developed. The approach combined the mass loss rate model with the mixed rule formulation. The elastic modulus of C/SiC composites at 400~700 ℃ and 700~900 ℃ air environment which represents two different kinds of oxidation model was predicted. The influence of oxidation temperature, oxidation time and fiber volume fraction on composites elastic modulus was analyzed. The experiments of C/SiC composites at 650 ℃ and 800 ℃ air environment with non-stress have been performed, the relationship between mass loss rate and oxidation time has been established, and the tensile stress-strain curves have been obtained. Besides, the theoretical results have been compared with the experimental data. It is found that the predicted results agree well with the experimental ones. And it is verified that the approach developed in this paper can predict the elastic modulus of ceramic matrix composites under non-stress oxidation efficiently.