In this paper, the synergistic effect of the thermally conductive networks in polyethylene (PE) composites was investigated. When the filler ratio was boronnitride (BN): aluminum nitride (AlN): aluminum oxide(Al2O3)=20:5:5, the PE/hybrid fillers (BN/AlN/Al2O3) composites had the highest thermal conductivity. It demonstrated the synergistic effect of the hybrid fillers without whiskers of the fillers in the PE composites. The gel point showed the φc (PE/Al2O3)=37.4 and the φc (PE/BN= 42.7 through heological measurement. Simultaneously, the gel point of the PE/hybrid fillers composites was not observed during the whole measurement range. However, the PE/hybrid fillers composite had the highest thermal conductivity than other composites. As a comparative investigation, the gelpoint of the PS/BN composites was studied, and the results showed that the φc (PS/BN) was lower than the φc (PN/BN). However, the thermal conductivity of the PE/BN composite was higher than that of the PS/BN composite at the same BN content. This result also indicated that additional thermally conductive paths, which were built in form of the filler–crystal–filler, may exist in the PE/BN composites. Therefore, the synergistic effect of the filler–crystal–filler network and fillers particle networks resulted in higher the thermal conductivity of the PE/BN composite in comparison with that of the PS/BN composites.