This work offers a simple fabrication of porous boron-nitride-(BN)/polyimide-(PI) composite films with high thermal diffusivities and low dielectric constants by combining high-internal-phase Pickering emulsification (HIPPE) and subsequent hot-pressing. BN nanoparticles in composite foams were well dispersed and three-dimensionally connected following the surface of the PI skeleton used as the polymer matrix. The BN contents in the BN/PI composite foams were adjusted in the range of 20-80 wt%. The porosities of the composite films were controlled according to the hot-pressing conditions such as the temperature. The porous BN/PI composite films exhibited high thermal diffusivities of 0.059-1.033 mm(2)/s and low dielectric constants of 2.08-3.48 at 1 GHz for BN contents of 20-80 wt%. In particular, the BN/PI composite films had extremely low dielectric loss values, close to zero (<0.002) at high frequencies regardless of the BN content and pressing conditions. (C) 2019 The Korean Society of Industrial and Engineering Chemistry.