Double-scale model for three-dimension-4 directional(3D-4d) braided C/SiC composites has been proposed to investigate its elastic properties. The double-scale model involves micro-scale that takesfi ber/matrix/porosity infi bers tows into consideration with unit cell which considers the 3D-4d braiding structure. Micro-optical photographs of composites have been taken to study the braided structure. Then a parameterized fi nite element model that refl ects the structure of 3D-4d braided composites is proposed. Double-scale elastic modulus prediction model is developed to predict the elastic properties of 3D-4d braided C/SiC composites. Stiffness and compliance-averaging method and energy method are adopted to predict the elastic properties of composites. Static-tension experiments have been conducted to investigate the elastic modulus of 3D-4d braided C/SiC composites. Finally, the effect of micro-porosity infi bers tows on the elastic modulus of 3D-4d braided C/SiC composites has been studied. According to the conclusion of this thesis, elastic modulus predicted by energy method and stiffness-averaging method bothfi nd good agreement with the experimental values, when taking the micro-porosity infi bers tows into consideration. Differences between the theoretical and experimental values become smaller.