Highly oriented and columnar grained diamond layers only a few microns thick, deposited at different substrate temperatures (500, 550, and 800 °C) on silicon using microwave-plasma-assisted chemical vapor deposition, are investigated by special photothermal techniques and high-resolution transmission electron microscopy (HRTEM). Small effective diamond–silicon boundary resistances of<4×10−9 m2 K/Ware determined for thermal conduction normal to the interface. Thermal conductivities normal to the interface,k⊥,are found to be about an order of magnitude greater than the conductivities parallel to the interface,k∥(k⊥/k∥=9–18).The boundary resistances measured are in good agreement with limits estimated from the interface structure observed by HRTEM, which indicate a low near-interfacial disorder for the layers. ©1997 American Institute of Physics.