A study was carried out to evaluate the thermal conductivity of polybutylene terephthalate (PBT) filled with different grades of boron nitride (BN) fillers. BN of various particle sizes and surface area as well as those with and without surface modification were used. The effect of particle size, surface area, surface modification and concentration on the thermal performance of the composites was discussed. An evaluation was also made on then-no-mechanical properties and rheological behaviour of the various composites.