Flaky hexagonal boron nitride particles-reinforced epoxy composites were prepared. The investigation on the thermal, electrical, and mechanical properties of hexagonal boron nitride/epoxy composites indicated that the incorporation of silane coupling agent-treated hexagonal boron nitride into epoxy slightly improved the glass transition temperature and the thermal stability of the composites; the epoxy containing silane treated hexagonal boron nitride exhibited higher thermal conductivity compared to the untreated ones. The dielectric permittivity increased slowly with an increase in hexagonal boron nitride content, as well as with a decrease in frequency. The obtained 50 wt% hexagonal boron nitride–filled epoxy composite had a low dielectric permittivity (less than 5.4) and dielectric loss (less than 0.02) in all frequencies ranging from 10⁻¹ to 10⁷Hz, a high volume resistivity of 6.3 × 10¹⁴ Ω·cm, and a high dielectric strength of 16 kV/mm, together with moderate mechanical properties, which are of great significance for practical electrical materials applications.