High thermal-conductivity fillers of aluminum nitride (AlN) and boron nitride (BN) were incorporated in the epoxy matrix in order to identify the effects of the particle size and the relative composition on the thermal conductivity of composites. In the bimodal distribution of polygonal AlN and planar BN particles, the optimal thermal conductive path was strongly affected by the packing efficiency and interfacial resistance of the particles in a sensitive way and, consequently, the maximum thermal conductivity was achieved up to 8.0 W/mK in the 1:1 volume ratio of AlN:BN particles. In the optimal volume ratio of the two fillers at 1:1, the relative filler size, which was represented by the shape factor (or the diameter ratio of the two filler particles, R-D), also influenced the thermal conductivity giving the maximum conductivity at the shape factor R-D approximate to 1. The optimal morphology and composition of the AlN/BN composite systems were clearly visualized and thoroughly discussed in the filler distribution curves plotting the filler-appearance frequency as a function of particle size. The developed methodology validated that two different particles should be packed well to fill up the interstitial space and, simultaneously, the contact resistance and the contact area of the fillers should be optimized to maximize the thermal conductivity. (C) 2012 Published by Elsevier B.V.