In this study, the microstructural development and its effect on the thermal conductivity of polyamide /cyclic olefin copolymer (PA/COC) blends containing boron nitride (BN) particles were investigated. The co-continuous morphology of the BN-filled composites was confirmed by SEM images. The composite samples were prepared by a masterbatch method in order to localize the BN particles in the PA phase. The melt viscoelastic results have shown a liquid-solid transition at low-frequency range, indicating a three-dimensional percolated microstructure. However, the results have not shown a pronounced percolation in the thermal conductivity. These results have suggested that the sensitivity of the thermal conductivity of these composites to percolated microstructure is not as strong as the rheological behavior. A comparison made between the BN-filled PA and the blend composites with co-continuous morphology revealed a lower thermal conductivity for the PA-BN/COC compared to the PA/BN composites with the same BN concentration. Moreover, the values of thermal conductivity for the blend composites were found to be comparable with those predicted by a combination of Nielsen's model (for filled polymers) and Wang's model introduced for blends with co-continuous morphology.