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The core and skin microstructure of T700 carbon fibers in carbon/carbon composites, prepared with chemical vapor infiltration (CVI) at 1000 degrees C and heat treated at 2300 and 2800 degrees C, have been studied by means of high-resolution transmission electron microscopy (HRTEM). The orientation angles of the graphitic basal planes obtained from selected-area electron-diffraction patterns show that the structure difference between the core and skin is a change of the degree of preferred orientation of the graphitic basal planes which decreases gradually from the skin region to the core region after CVI at 1000 degrees C. With increasing heat-treatment temperature, the basal planes orient parallel to the fiber axis, first in the skin region and then in the core region. In addition, the diameter of the core region decreases considerably from about 3.3 to 2.2 mu m after heat treatment at 2800 degrees C. HRTEM lattice-fringe images show that the graphitic crystallite size increases significantly both in the core and skin, but more in the skin. Moreover, with increasing crystallite size, pores of nanometer scale start to form in the fiber. (C) 2011 Elsevier Ltd. All rights reserved.