C/SiC composites were multi-phase materials consisting of C fibers， SiC matrix， and a thermally decomposed carbon interface layer between the two. This paper analyzed and tested the microstructure and nano-mechanical properties of the fibers， matrix， and interface， thereby complementing the results of macro-mechanical property tests. Optical microscopy （OM）， scanning electron microscopy （SEM）， and energy dispersive spectroscopy （EDS） were employed to characterize the refined microstructure of the C/SiC cross-section. Nanoscratch testing was conducted to measure the hardness and equivalent modulus of representative regions. The results reveal that the C fibers in the C/SiC composite exhibit a “skin-core” structure， with a diameter of approximately 7 μm. The ring-shaped region within a 1.5 μm range of the outer layer has a hardness of （5.45 ± 0.28） GPa and a reduced modulus of （33.1 ± 1.4） GPa； the core region has a hardness of （6.85 ± 0.21） GPa and a reduced modulus of （33.8 ± 0.34） GPa. The SiC matrix comprises three forms： large matrix blocks distributed between fiber layers； small matrix blocks distributed around the large matrix blocks； and fine matrix particles distributed within individual fiber bundles and between different fiber strands. Due to the differing high-temperature pyrolysis environments experienced by these three types of matrix， there are significant differences in hardness and reduced modulus， with hardness values of （34.8 ± 2.64）， （23.6±2.27）， and （21.3±1.81） GPa， respectively， with reduced modulus of （210±19.7）， （165±8.58）， and （124±10.8） GPa， respectively. The interface layer thickness between C fibers and the SiC matrix is approximately 0.3 μm. Its mechanical properties exhibit significant variability， lying between those of the fibers and the matrix， thereby serving as a performance transition layer. Its hardness is （13.6 ± 3.03） GPa， and its reduced modulus is （99.3 ± 13.0） GPa.