In summary, the epoxy/BNNT-BNNS nanocomposites with high thermal conductivity and excellent thermal stability are successfully fabricated. The composites exhibit a greatly enhanced in-plane thermal conductivity of 2.6 Wm -1 K -1 and 3.4 Wm -1 K -1 respectively, which is 10 to 17 times higher than the thermal conductivity of the pristine epoxy, when BNNS/BNNT volume fraction is 30% and 1% in epoxies LY564 and LY5052 respectively. This is mainly due to the uniform dispersion and formation of covalent bond between fillers in the epoxy resin, which creates an effective heat conduction network. The hybrid composites demonstrate a synergistic improvement in thermal conductivity, which can be attributed to the development of a three-dimensional thermal network between BNNTs and BNNSs. However, the excess addition of BNNT in the composite, which cannot contribute to the inter-connectivity between BNNS materials, led to increased phonon scattering density, resulting in decreased thermal conductivity. This research provides new insights into the design, fabrication, and improvement strategies for polymer-based composites with excellent thermal conductivity.