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The SiC/SiC composite has been proposed as a candidate material for the fuel cladding component in advanced nuclear reactors. The present thesis aims to characterize the damage evolution within a SiC/SiC composite in different conditions and then validate a multiscale damage model. In order to gain insight into the damage mechanisms, in situ experimental methods are required to detect damage development and quantify the mechanical response at different scale to support the reliability assessment and simulation modelling of the components. In this thesis, in situ 3D laboratory and synchrotron X-ray computed tomography and quantitative digital volume correlation have been applied to characterize the damage evolution in a SiC/SiC composite, with respect to its heterogeneous microstructure and macroscale mechanical response. The quantified information from tomography further helps the development of an advanced multiscale model with the microstructure sensitivity. Experimental methods for the direct in situ observation of damage have been developed for SiC/SiC C-ring specimens at both room temperature and elevated temperature. The time-resolved tomography allows the continuous observation of the damage within the heterogenous microstructure and the macroscale deformation. The development of the damage in the composite depends on the loading history, and the high temperature (900°C) in air degenerates the mechanical properties of the SiC/SiC composite as shown in the observations of the crack paths. The hoop pressurization test of the SiC/SiC composite tube via an expanding plug revealed the distribution of the matrix cracks at both the interior and outer surfaces of tube sample for the first time. The full-field multiscale in situ observation quantifies the non-uniform deformation of the composite tube under internal pressure with the corresponding strain measurement. It also allows a more accurate experimental measurement of the mechanical response in the composite tube component. Damage simulations have been performed in the finite-element microstructure meshfree (FEMME) model. The 3D experimental observation provided the statistical description of the microstructure for the model. The virtual damage simulated by the FEMME model is compared with the experimental observations in both the C-ring and hoop pressurization tests. The capability of the microstructure sensitive model has been demonstrated with a high fidelity of the simulated damage evolution and mechanical response in the composite. This coupled experimental and simulation studies could provide a more comprehensive understanding of the damage mechanisms in the SiC/SiC composite.