In this work, the thermal conductivity of composites made up of carbon nanotubes embedded in a polyester resin is investigated by comparing experimental data with theoretical predictions. The composite samples were prepared with a random and aligned distribution of carbon nanotubes. Its thermal conductivity is then measured by using the photothermal radiometry technique. The obtained experimental data is accurately described by the proposed theoretical model, which takes into account the size effects of the nanotubes. It is expected that the obtained results can provide useful insights on the thermal design of composites based on carbon nanotubes.