Heat dissipation is one of the most important issue in modern highly integrated electronic devices. Thermal conduction in semiconductors becomes less efficient as structures scale down to nanoscale due to the predominant phonon-boundary scattering. Besides the classical heat dissipation channels of conduction, convection, and radiation, recent studies have indicated that surface phonon-polaritons (SPhPs), which are evanescent electromagnetic waves generated by the hybridization of optical phonons and photons, open a new heat transport channel in nanofilms [ Fig. 1(a) ] [1] - [4]. These waves propagate along the surface of polar dielectric materials and could be powerful heat carriers capable of enhancing the thermal performance of micro- and nanoscale devices. In this work, we experimentally observe the energy contribution of SPhPs by systematic measurements of the in-plane thermal conductivity of SiN nanofilms with different thicknesses and temperatures ranging from 300 to 800 K.