<p>As the heat generated per unit volume would continue to rise, the thermal conductive polymers are commonly prepared through filling high weight content of functional fillers, which often resulted in the deterioration of dielectric property. It is an urgent issue to balance the thermal conductivity and dielectric property. In this work, carboxylated boron nitride nanosheets (BNNS) were exfoliated from ball milling wrapped aminated polyurethane (PU) network by chemical bonding. Composites of epoxy resin, BNNS, and PU (BNNS@PU/EP) were successfully prepared through hot-pressing three-dimensional network structure of BNNS/PU. Thermal conductivity, insulation, and dielectric property of BNNS@PU/EP were systematically studied. Thermal conductivities of BNNS@PU/EP reached 0.47 W/m&#xa0;K (4.39 wt% of BNNS) and 0.519 W/m&#xa0;K (5.88 wt% of BNNS) with the percentage improvement (per 1 wt% of BNNS) of 36.12 and 30.91, respectively. Given the excellent insulation performance and low filling content of BNNS, BNNS@PU/EP (5.88 wt%) still maintained a high volume resistivity of 8.06 × 10¹⁴ Ω&#xa0;cm and an alternating current breakdown field strength of 79.86&#xa0;kV/mm. The obtained dielectric constant was less than EP at 10–10⁶&#xa0;Hz, and the dielectric loss was 0.0072 at 10⁶&#xa0;Hz. Our study paved a way for the fabrication of ceramics-filled polymers with enhanced thermal conductivity and excellent dielectric property at low filling contents.</p> Graphical abstract <p></p>