We prepared polypropylene (PP) filled with boron nitride (BN) particles by melt compounding. The effects of filler content, particle size, and combined filler on the thermal conductivity of PP/BN composites were investigated. The results indicated that the thermal conductivities depend on the content and BN particle size. The good dispersion of BN particles of intermediate size in PP matrices led to a higher thermal conductivity than that of PP matrices containing small and large BN particles in which the agglomeration of BN particles was observed. The incorporation of BN particles of two different sizes could enhance the thermal conductivities of PP composites relative to those of composites containing BN particles of a single size at the same total content. A PP composite containing 50 wt% combined fillers with a mixture of BN particles measuring 45 mu m and 10 mu m at a weight ratio 1 : 4 showed the highest thermal conductivity in this experiment. Furthermore, several models for predicting the thermal conductivity of the composites were studied.