Ideal materials for modern electronics packaging should be highly thermoconductive. This may be achieved through designing multifunctional polymer composites. Such composites may generally be achieved via effective embedment of functional inorganic fillers into desirable polymeric bodies. Herein, two types of high-performance 3D h-BN porous frameworks (3D-BN), namely, h-BN nanorod-assembled networks and nanosheet-interconnected frameworks, are successfully created via an in-situ carbothermal reduction substitution chemical vapor deposition using carbon-based nanorodinterconnected networks as templates. These 3D-BN porous materials with densely-interlinked frameworks, excellent mechanical robustness and integrity, highly-isotropous and multiple heat transfer paths, enable reliable fabrications of diverse 3D-BN/polymer porous composites. The composites exhibit combinatorial multi-functional properties, such as excellent mechanical strength, light weight, ultra-low coefficient of thermal expansion, highly isotropic thermal conductivities (~ 26 -This article is protected by copyright. All rights reserved.3 51 multiples of pristine polymers), relatively-low dielectric constants and super-low dielectric losses, and high resistance to softening at elevated temperatures. In addition, the regarded 3D-BN frameworks are easily recycled from their polymer composites, and may be reliably reutilized for multi-functional reuse. Thus, these materials should be valuable for new-era advanced electronic packaging and related applications.