The effects of the permittivity anisotropy and size of a nano thin film on its thermal conductivity due to surface phonon-polaritons are studied. We demonstrate that this thermal conductivity is a linear combination of the inverse first and third powers of the film thickness. For a 100-nm-thick film of α-quartz surrounded by air, the thermal conductivity along the optical axis is comparable to the phonon counterpart and equals 13 W m %1 K %1 , which is 25% higher than that along the perpendicular direction, at room temperature. Higher values are found for thinner films at higher temperatures. S urface phonon-polaritons (SPPs) are electromagnetic waves generated by coupling between photons and phonons at the interface between two media. 1–3) Over the past few years, various research groups have shown that these surface waves have promising applications for improving the thermal performance of nanoscale devices, 4–6) radiative heat transfer, 7–10) high-density infrared data storage , 11) surface infrared absorption, 12) coherent thermal emission, 13) and photonics. 14,15) In these cases, the operating principles are based on the fact that in nanomaterials, the surface effects predominate over the volumetric ones, so the energy transport by SPPs is particularly important. The SPP energy contribution increases as the material size is scaled down in the direction perpendicular to the propagation one. 4,6)