Abstract Carbon nanotubes are considered to be ideal candidates for matrix reinforcement in fibre-reinforced composite materials. In order these new multifunctional materials to be used at their optimum potential, precise measurements are completely essential. This article is focused in the accurate measurement of the enhancement of the thermal conductivity of an epoxy-resin, reinforced initially with plies of plain weave glass fabric then by carbon multi-walled nanotubes (C-MWNT), and finally with both these two macroscopic and nanoscopic reinforcements at hand. The technique employed was the transient hot-wire technique, as it was recently modified to be able to measure the thermal conductivity of solids in an absolute way, with an uncertainty of better than 1%. Following validation of the technique, the results revealed that in the case of reinforcing the epoxy with glass fibres, with volume fraction of 28%, the thermal conductivity increase was 27% compared to plain epoxy-resin. When reinforced with 2% by weight C-MWNT the enhancement was 9% and when reinforced with both the C-MWNT and glass fibres the enhancement was the highest value obtained, being 48%.