While carbon nanotubes (CNT) have been suggested as thermal management material for integrated circuits, the thermal properties, and, especially, the thermal boundary resistance (TBR) of as-grown CNT fabricated at low temperature have hardly been investigated. Here, the thermal resistance of CNT vias, with different bundle lengths and diameters fabricated at 500 °C using chemical vapour deposition, are investigated using the 3ω-method. It was found that the thermal resistance hardly changes with length except for the smallest bundle diameter of 2 μm, indicating that the TBR (109–1010 K/W per tube) dominates the thermal conduction. This is in contrast to the electrical resistance and temperature coefficient of resistance, both of which clearly increase with length. From the slope of the thermal resistance versus length of the 2 μm wide bundles, the thermal conductivity of the CNT was estimated to be 1.4–2.8 W/mK. This low thermal conductivity is attributed to the low quality of the samples as determined by Raman spectroscopy.