Multiwalled carbon nanotubes (MWCNTs) were oxidized using four different acid-treatment methods followed by their functionalization with 3-aminopropyltriethoxysilane (3-APTES). Diglycidyl ether of bisphenol A (DGEBA) nanocomposites with unmodified and silanized MWCNTs (0.2 wt %) were prepared by a cast molding method. The effect of functionalization of MWCNTs on thermal, flexural, and morphological properties of the epoxy nanocomposites were studied. The epoxy/MWCNTs nanocomposites were characterized by thermogravimetric analysis, flexural testing, and field emission electron microscopic analysis. The results showed that the silanization of MWCNTs which were oxidized by a two-step process using nitric acid and hydrochloric acid showed better thermal and flexural properties due to good interfacial adhesion between MWCNTs and the epoxy matrix. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1175-1184, 2010