In this paper, we considered air void as the key parameter to improve thermal conductivity prediction model. Influences of air void on thermal conductivity of Polymer/BN nanocomposite was investigated by FEM based simulation. Thermal conductivity of composite was decreased with increase of air void content. In case of BN filler with 10 percent of volume fraction and thin layer air void model, thermal conductivity was decreased about 12 percent when we add air void with 1 percent of void content. Position and distribution of air void also influenced thermal conuductivity of nanocomposite. Thermal conductivity of nanocomposite decreased with an increase of contact surface area between air void and filler particle. For verifying these simulation results, we made Polymer/BN nanocomposite samples and analyzed their air void content by applying BET method and thermal conductivity by laser flash method. Influence of content and position of air void is not neglectable especially when the rate of formation of air void is high or air void formed between filler and matrix in the process of composite fabrication. If the parameters related to air void considered in thermal conductivity prediction model, prediciton model will be improved.