Epoxy resin as the underfill material were widely used in microelectronic packaging, and the heat transfer performance has become an important factor affecting IC integration development. In this work, using the molecular dynamics, the thermal and rheological properties of BNNS/epoxy composites has been analyzed, and the mechanism of BNNS in enhancing the thermal conductivity of composites has been elaborated. Firstly, the models of neat epoxy and BNNS/epoxy composites were constructed, the glass transition temperature and coefficient of thermal expansion were calculated. Then, based on the Green-Kubo theory, equilibrium molecular dynamics (EMI)) was used to simulate the thermal conductivity of the BNNS/epoxy composites. Furthermore, considering the flow performance of BNNS/epoxy composites, the viscosity was analyzed. Finally, the influence of BNNS loading and size on thermal and rheological properties of BNNS/epoxy composites were also studied. The results show that adding BNNS effectively improved the thermal conductivity of the composites, but also significantly increased the viscosity. When the size of BNNS increases, the thermal conductivity of composites increases and the viscosity decreases. This work will provide microscopic views and insights into the thermal and rheological mechanism of BNNS/epoxy composites and offer effective guidance to design and prepare high thermal conductivity, low viscosity underfill materials. (C) 2018 Elsevier Ltd. All rights reserved.