A thermal conductivity of 32.5 W/mK is achieved for a boron nitride-filled polybenzoxazine at its maximum filler loading of 78.5% by volume (88% by weight). The extraordinarily high conductivity value results from outstanding properties of the polybenzoxazine matrix and the boron nitride filler. The bisphenol-A-methylamine-based polybenzoxazine possesses very low A-stage viscosity which aids in filler wetting and mixing. The filler particles with an average size of ca. 225 mu m are large aggregates of boron nitride flake-like crystals. It has bimodal particle size distribution which assists in increasing the particle packing density. This filler-matrix system provides a highly thermally conductive composite due to the capability of forming conductive networks with low thermal resistance along the conductive paths. The SEM picture of the composite fracture surface reveals good interfacial adhesion between the boron nitride filler and polybenzoxazine matrix. Water absorption of the filled systems at 24 h is <0.1% and decreases with increasing filler content.