Abstract: The high density integration of components in electronic and electrical equipment makes the problem of heat dissipation increasingly prominent, and the demand for thermal conductive materials is increasing. In this paper, polyethylene terephthalate (PET) and hexagonal boron nitride (h-BN) were used as the matrix and thermally conductive filler, respectively, a series of h-BN/PET composites were prepared by melt blending method. The effect of the h-BN content and the crystallinity of the PET matrix on the thermal conductivity of the composites were investigated, and the thermal conduction mechanism of the composites was analyzed. The temperature dependence of the thermal conductivity and heat dissipation effect of the composites were explored from the perspective of material application. The results show that both the crystallinity of the PET matrix and the content of h-BN both contribute to the final thermal conductivity of the composites, and the thermal conductivity of the composites increases with the increase of the crystallinity and h-BN content. h-BN plays a role of heterogeneous nucleation, significantly accelerates the crystallization rate of PET and improves the crystallinity of PET. In compression molding, h-BN is driven by shear stress to oriented in the direction of flow in the PET matrix, resulting in the composite material showing obvious anisotropic characteristics. The orderly arrangement of h-BN in the in-plane direction provides more channels for phonons transmission. When the filling amount of h-BN reaches to 50wt%, the in-plane and through-plane thermal conductivity of the composites reach the maximum values of 3.00 W·(m·K)⁻¹ and 2.19 W·(m·K)⁻¹, respectively. h-BN/PET composites have good heat dissipation effect. The higher the h-BN content, the faster the cooling rate. The rule of temperature drop conforms to the exponential function during the heat dissipation process.