Abstract <p>One of the main problems limiting effective operation of electronic devices is overheating. In order to provide effective heatsink in many applications, dielectric polymer composite materials should be used. Excellent dielectric and thermal properties, minimal thermal expansion considered, they should also provide stable properties throughout all temperature range of operation. In the present paper, temperature dependencies (25–200°C range) of thermal conductivity, heat capacity, thermal diffusivity, and thermal expansion coefficient of hexagonal boron nitride/phenol-formaldehyde resin are considered. Most important dielectric properties—permittivity and dielectric loss tangent, were also measured. Filler volume fraction varied from 5 to 85%. Thermal diffusivity at highest filler fraction was 3.4 mm²/s (25°C, ca. 2300% improvement over neat polymer), thermal conductivity—7.5 and 16.4 W/(m K) in orthogonal directions being stable up to 200°C. Agari-Uno model parameters (including filler anisotropic thermal conductivity) best fitting the system under study have been calculated. Conclusions on material composition providing optimal thermal, dielectric and thermo-mechanical properties for heatsink bulk thermal management electronic-grade composite, are made.</p>