The thermal conductivity of polyimide/boron nitride (PI/BN) nanocomposite thin films has been studied for two sizes of BN nanofillers (40 and 120 nm) and for a wide range of content. A strong influence of BN particle size on the thermal conduction of PI has been identified. In the case of the largest nanoparticles (hexagonal-BN), the thermal conductivity of PI/h-BN (120 nm) increases from 0.21 W/mK (neat PI) up to 0.56 W/mK for 29.2 vol %. For the smaller nanoparticles (wurtzite-BN), PI/w-BN (40 nm), we observed two different behaviors. First, we see a decrease until 0.12 W/mK for 20 vol % before increasing for higher filler content. The initial phenomenon can be explained by the Kapitza theory describing the presence of an interfacial thermal resistance barrier between the nanoparticles and the polymer matrix. This is induced by the reduction in size of the nanoparticles. Modeling of the experimental results allowed us to determine the Kapitza radius a K for both PI/h-BN and PI/w-BN nanocomposites. Values of a K of 7 nm and >500 nm have been obtained for PI/h-BN and PI/w-BN nanocomposite films, respectively. The value obtained matches the Kapitza theory, particularly for PI/w-BN, for which the thermal conductivity is expected to decrease compared to that of neat PI. The present work shows that it seems difficult to enhance the thermal conductivity of PI films with BN nanoparticles with a diameter <100 nm due to the presence of high interfacial thermal resistance at the BN/PI interfaces. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42461.