Abstract: The worn surfaces of two kinds of carbon/carbon composites after 10 time simulated braking tests of an aircraft were characterized by means of micro Raman spectroscopy and X ray diffraction. The matrix of the composites consists exclusively of pyrolytic carbon or a combination of carbonized pitch and pyrolytic carbon. The structural changes of the carbon on the worn surfaces of the composites were analyzed, and the effect of the structural changes on the friction and wear behavior of the composites discussed as well. As the results, the difference in the graphitization degree of the carbon fibers in the sub worn surface and the matrix becomes indistinctive on the worn surfaces after 10 time simulated braking testing, the size of the microcrystalline graphite along axial a becomes uniform as well. In other words, with respect to the reciprocal of the integral intensity ratio R of the peaks at 1 338.94 cm -1 and 1 585.22 cm -1 in the Raman spectra of the worn composite surfaces, the value of 1/ R for the components of lower graphitization degree increases considerably after the simulated braking test, while that for the components of higher graphitizatino degree decreases to some extent. At the same time, the graphite crystals on the worn surface turn gradually direction until their c axial comes to vertical to the worn surface, and finally the crystals in flake shape cover the worn surface in the presence of normal stress. Subsequently the worn surfaces of almost the same composition and grapgitization degree were generated for the C/C composites of different structures and densities. This accounts for the insensitivity to the structure and density of the friction and wear behavior of the composites.