Colloidal nano-silica particles were used to improve the scratch and mar resistance of waterborne epoxy coatings by directly blending. To enhance the compatibility of nano-silica particles within polymer matrix, nano-silica particles were first modified with 3-glycidoxy-propyl-trimethoxysilane (GPTMS) and characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The modified nano-silica particles were found to have a better dispersion and compatibility in the polymer matrix than the unmodified nano-silica particles. Macro-scratch and nano-scratch testers were employed to characterize the scratch and mar resistances of the nanosilica-reinforced epoxy coatings. Relative to unmodified nano-silica, GPTMS-modified nano-silica particles can improve the scratch and mar resistance more significantly and reduce the transparency and gloss of waterborne epoxy coatings less seriously. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3634-3639, 2009