Abstract: Air bubbles and gaps in epoxy matrix composites seriously influence its thermal conductivity. Research on the effect of bubbles and gaps on the thermal conductivity of composites benefits to improve the accuracy of thermal conductive model and provides guidance for optimization of thermal conductivity. A numerical model of hexagonal boron nitride (h-BN)/epoxy composites with air bubbles and gaps was established by finite element method. The effects of bubble size and number, gap size and number on the thermal conductivity of h-BN/epoxy composites were systematically analyzed. This model was validated by other thermally conductive models and experimental data. The results show that with the increase of bubble size and number, the thermal conductivity of h-BN/epoxy composites decreases gradually, and a turning point occurs in the thermal conductivity curve changing with bubble size. Bubbles with diameter greater than the thickness of unit cell have a great impact on the thermal conductivity of h-BN/epoxy composites. With the increase of gap diameter and thickness, the thermal conductivity of h-BN/epoxy composites decreases slowly first, and then fast. Finally, the thermal conductivity of h-BN/epoxy composites decreases linearly. With the increase of the gap number, the thermal conductivity of h-BN/epoxy composites decreases gradually. The air gaps at the interface of h-BN and epoxy have a greater influence than those in the epoxy matrix.