The continuous development of electronic devices (The demands in denser & faster circuits intensify) & miniaturization of microelectronic devices to boost their power (hinges on the development of) practical solutions for the management of excessive amount of heat generation. This requires new polymer- matrix composites (PMCS), with good processability, high effective thermal conductivity (K), & low but tailored electrical conductivity. In this article the thermal conductivity of composites with a poly polybutylene terephthalate (PBT) matrix & a mixture of Boron nitride (BN) powder & multi-wall Carbon nanotube (MWCNT) fillers were investigated. Synergistic improvement in thermal conductivity of the composite was observed due to the generation of three dimensional Thermal transfer pathways between the BN & MWCNT. The thermal conductivity of the composite was affected by the interaction & interfacial thermal resistance between the PBT matrix and fillers. The maximum thermal conductivity achieved was 1.46 for a composite contains %5wt CNT.SEM images showed that the BN dispersed well, but CNTs agglomerated in some ways. Based on DSC data, the fillers had little effect on the melting behavior of the composites, but a prominent heterogeneous nucleation effect and greatly accelerated its crystallization procedure of PBT. The melt rheology analysis further confirmed the filler concentration for conductive network formation of BN.