In this article, nano-zinc oxide (ZnO) filled ethylene propylene diene monomer (EPDM) composites are prepared, and the mechanical (static and dynamic) properties and thermal conductivity are investigated respectively, which are further compared with the traditional reinforcing fillers, such as carbon black and nano-silica. Furthermore, influence of in-situ modification (mixing operation assisted by silane at high temperature for a certain time) with the silane-coupling agent Bis-(3-thiethoxy silylpropyl)-tetrasufide (Si69) on the nano-ZnO filled composites is as well investigated. The results indicate that this novel reinforcing filler nano-ZnO can not only perform well in reinforcing EPDM but can also improve the thermal conductivity significantly. In-situ modification with Si69 can enhance the interfacial interaction between nano-ZnO particles and rubber matrix remarkably, and therefore contribute to the better dispersion of filler. As a result, the mechanical properties and the dynamic heat build-up of the nano-ZnO filled composites are improved obviously by in-situ modification, without influencing the thermal conductivity. In comparison with traditioanl reinforcing fillers, in-situ modified nano-ZnO filled composites exhibit the excellent performance in both mechanical (static and dynamic) properties and better thermal conductivity. In general, our work indicates that nano-ZnO, as the novel thermal conductive reinforcing filler, is suitable to prepare elastomer products serving in dynamic conditions, with the longer expected service life. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1144-1155, 2011