The SiC/SiC composites are considered fiber bundle-reinforced SiC matrix composites at the mesoscale, where each fiber bundle itself is a unidirectional SiC fiber-reinforced SiC matrix composite. In this chapter, addressing the nonlinear mechanical behavior and multiscale failure modes of SiC/SiC composites, the micro- and mesoscale damage constitutive models are developed and integrated into a defect-related multiscale simulation framework, enabling the prediction of mechanical response at the SiC/SiC fiber bundle and material levels. By introducing corresponding damage variables, the probabilistic fracture of the SiC fibers at the microscale and the multiple damage modes of the fiber bundles at the mesoscale are characterized, respectively. For the SiC matrix spanning from micro- to mesoscale, a damage constitutive model incorporating multiple deformation mechanisms is developed, accounting for linear elasticity, microcrack-induced damage, and inelastic behavior. Furthermore, simulation implementations are presented that illustrate the algorithm procedure and the convergence rate of the multiscale models in the context of the finite element method. These simulations can be used to predict the key deformation, damage evolution, and failure envelopes of the multiscale representative volume elements (RVEs) under uniaxial and combined loadings.