The present paper intends to study the microstructure of vapour grown carbon fibres (VGCF), whose fracture depends on the breaking mode, as it was stated. Before being examined, transversal sections of fibres were etched, using an argon ion beam, aiming to remove a thin surfacial layer, then seriously altered by the failure process. After that, based upon the results of the scanning electron microscopy (SEM) examination, it can be established that the inner core of as-grown VGCF is constituted by a Rowland＇s structure. Although differentiated crystals do not exist in the cortical phase, the boundary between the core and the latter phase is clearly observable. After that, an overhydrogenation process took place as a result of an annealing of the fibres in a hydrogen atmosphere.The microstructure had been, by then, slightly modified. In the inner core, not only the gap width between crystals had been modified but the boundary between catalytical and pyrolytical phases had vanished as well. As a consequence of such modifications, the fibres Young＇s modulus increases and their failure strain diminishes, in a similar manner as the mechanical properties of current ex-polyacrylonitrile (PAN) carbon fibres are altered by graphitization, during a graphitizing process.