Polymer coating with high thermal conductivity is of great value in the field of heat dissipation for high-performance electronic devices. In this paper，a UV-curable composite coating was prepared by blending functionalized thermal conductive fillers of different sizes and concentrations with photocurable resin. Firstly，using acrylated cashew phenol as a functional additive，the exfoliation and functionalization of hexagonal boron nitride nanosheets（CBNNS）of different sizes were achieved through mechanical ball milling. Then， two different sizes of CBNNS were used as mixed thermal conductive fillers to prepare UV-curable composite coating. By changing the ratio of different size CBNNS and adjusting the content of mixed fillers，the effects of filler size and content on coating properties were systematically studied. The results showed that when the mass ratio of large and small fillers was 7.5∶2. 5，the thermal conductivity（TC） of the composite coating with a total filling amount of 20% could reach 3. 15 W/（m·K）， and the heat dissipation performance wassignificantly enhanced. The improvement of the thermal conductivity of the composite coatingcould be attributed to the high thermal conductivity of CBNNS，the low interfacial thermal resistance between functionalized fillers and resin matrix，the orientation of large-sized CBNNS，the synergistic effect of mixed fillers and so on.