Abstract: Nanoindentation method was used to test elastic modulus and nanohardness of different micro-areas of carbon fiber in 3D C/C composite, and the causes for differences mechanical in different micro-areas were analyzed. The results show that, under the load of 9.0, 9.5 mN, there was only elastic distortion happening in the center of carbon fiber, but the plastic deformation happened at the edge (distance from fiber center 2 μm). The values of elastic modulus and nanohardness in the center of carbon fieber are 33.42, 3.30 GPa, and at the edge of 32.41, 2.63 GPa. These differences of elastic modulus and nanohardness between edge and center were caused by the graphite microcrystallite arrangement differences