A morphological-structural investigation regarding the coating of vapour grown carbon fibers (VGCF) by polycrystalline diamond layers, produced by CVD activation of CH4/H-2 mixtures is reported. Transmission electron microscopy (TEM) and electron diffraction (ED) techniques allowed detailing, for the first time at nanoscale, the morphology and structure of different carbon layers across the 3-D geometry of the diamond coated fibers. Our results indicate an isotropic distribution of the various carbon phases along the direction transversal to the fiber axis, and the occurrence of turbostratic graphite layers at the fiber/diamond interface region. The diamond coating is formed by nano-grains with sizes ranging between 5 and 50 rim and substantially free of extended defects. (C) 2004 Elsevier B.V. All rights reserved.