Hexagonal boron nitride (hBN) has a structure similar to graphene and has high thermal conductivity, electrical insulation, and chemical stability. It is often used as a filler in polymer-based composites to improve thermal conductivity. hBN in the form of nanosheets and nanotubes also offers a promising alternative, as they have similar thermal conductivity to carbon nanotubes (CNTs) and graphene but also have a larger bandgap (4-6 eV), making them suitable for electronic devices that require both electrical insulation and high thermal conductivity. Previous studies reported that boron nitride nanotubes (BNNTs) had a high aspect ratio and high thermal conductivity ( 200 -300 Wm -1 K -1 ) , making them a suitable filler. Theoretical work has predicted that the thermal conductivity of a single-walled BNNT may even surpass that of CNTs. Therefore, this study designs a 3D filling strategy of BNNS/BNNT/epoxy composites to improve their thermal conductivity and examine the synergistic effect on the thermal conductivity of the composite. Small amounts of BNNTs are added to BNNS/epoxy composites by speed mixing process and the effect on thermal conductivity is evaluated by using laser diffusivity and novel modulated infrared (IR) thermoreflectance techniques. The method of creating these hybrid nanofiller-epoxy composites is simple, eco-friendly and can be easily scaled up, making them a potential option for use as TIMs in the next generation of electronic devices.