In this work different nano silica particles varying in aminosilane functionalities were dispersed into a polyurethane clear coating (PU). The optical clarity of the resulting films as well as the mechanical integrities were studied utilizing haze transmission and dynamic mechanical analysis respectively. In addition, the curing behavior of films was investigated using differential scanning calorimetry. It was found that particle dispersibility directly depended on the amount of grafted silane at the surface of nano silicas. Also the presence of amine functional groups on silica particles resulted in a systematic improvement of Tg and therefore in enhanced mechanical performance. These were attributed to an increased interfacial interaction and formation of some urea linkages in the hard segment of the polyurethane matrix. The DSC analyses also showed that the heat of reaction of particle loaded samples were all lower than that of the blank PU due to the facilitated interaction of amino groups at the silica surface with isocyanate cross-linker. (C) 2010 Elsevier B.V. All rights reserved.