The homogenized effective tensile moduli of 3D carbon/carbon composites (C/C) were determined by image-based finite element simulations and experiments. X-ray tomography was used to explore cracks, voids, and fiber bundles distortion and to reconstruct 3D finite element (FE) meshes containing most of these defects. Interfacial properties like debond strength and fracture energy in mode-II play an important role in the mechanical behavior of C/C composites. These properties were determined by conducting pull-out tests, which were also simulated using FE analysis, where frictional cohesive surfaces were used to model bundle/matrix interfaces. The properties of the cohesive surfaces were determined by comparing load displacement curves of FE simulations and pull-out tests. These properties were later used in the unit cells analysis with periodic boundary conditions to determine the homogenous moduli of the 3D C/C composite. The Young’s moduli obtained in y and z directions from simulations were compared to that obtained from experiments.