In this study, we examined the tensile and fracture behaviors of multi-walled carbon nanotube (MWCNT) reinforced epoxy nanocomposites with and without moisture absorption. The MWCNT/epoxy nanocomposites were fabricated using 0.1 wt.% unmodified, oxidized, and silanized MWCNTs and were kept in seawater for over 15 weeks. Silane-modified specimens demonstrated greater tensile strength, elastic modulus, and transmittance than unmodified or acid-modified specimens, irrespective of moisture absorption. Compared to dry nanocomposites, moisture absorption decreased the tensile strength and elastic modulus for each surface modi. cation. Fracture behavior showed similar tendencies as tensile test results. However, the fracture toughnesses of oxidized and silanized MWCNT/epoxy nanocomposites were not notably different, whereas unmodified specimens had much lower fracture toughnesses, irrespective of moisture absorption. Moisture absorption may have caused degradation resulting in weak interfacial bonding due to epoxy swelling. (C) 2010 Elsevier B.V. All rights reserved.