Abstract: The microstructure of 2D PAN carbon fiber-reinforced mesophase pitchbased matrix carbon/carbon(C/C) composites having a pyrocarbon transition layer was studied by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). PLM results indicate that the matrix consisted of pyrocarbon and mesophase pitch carbon, whose structure is anisotropic. TEM and SEM results show that multilayer interfaces are formed inside the composite. The carbon fiber-pyrocarbon interface is generally continuous, with a high degree of perfection and graphitic crystallites oriented parallel to the interface. The interface between the mesophase pitch carbon and the pyrocarbon is amorphous and is micro-fissured. The size and the degree of orientation of the graphitic crystallites gradually increase and the alignment of lattice fringes is gradually ordered from carbon fibers, to pyrocarbon to mesophase pitch carbon. The mesophase pitch matrix is lamellar carbon oriented parallel to the fiber axis. Numerous micro-cracks parallel to the carbon fiber axis are formed within the partially bonded interface and the matrix.