A protocol has been developed for the production of epoxy-based composites containing high-volume fractions of aligned carbon nanotubes. The nanotubes were fabricated as continuous fibres or aligned mats directly from the CVD reactor, in which they were synthesized. The block composites with highly aligned and tightly packed nanotube assemblies were prepared via epoxy resin infiltration, and their volume fraction, distribution, and internal porosity being analysed prior to mechanical testing. The samples were tested in both axial tension and three-point bending. The results show that the strength and stiffness enhancements were close to pro rata with the volume fraction of the carbon nanotubes added. The failure modes were distinctly different from those characteristic of the conventional aligned carbon fibre composites. The fracture surface showed considerable evidence of pull-out of bundles of (~50) nanotubes, but the pull-out appeared to involve the resin matrix which drew out along with the bundles. Subsidiary cracks were bridged by nanotube bundles giving structures reminiscent of crazes in glassy polymers, what constitutes the distinct toughness mechanism and higher resistance to the transverse cracks propagation.