The effects of the surface treatment of boron nitride (BN) particles on the thermal conductivity of BN/epoxy composite systems was investigated. By coating an amino silane compatibilizer on the BN surface, the interfacial space could be decreased so as to minimize the phonon scattering and thermal-interface resistance. When an excessive amount of silane compatibilizer was present at the BN/epoxy interphase boundary, it acted as a thermal insulation layer, resulting in the reduction of the thermal conductivity. Accordingly, the thermal conductivity was maximized when the optimal amount of silane compatibilizer was used, which was associated with the specific surface area or the size of the incorporated BN particles. In the case of the BN particles, whose specific surface areas were 14.3 m2/g and 11 m2/g (average particle size: 1 μm and 5 μm, respectively), the highest thermal conductivity was observed at 3.0 wt% and 2.5 wt% of the silane compatibilizer, respectively. By converting the particle size and specific surface area into the shape factor, the optimal amount of amino silane required to maximize the thermal conductivity was discussed in relation with the interphase structure and thermal resistance.