The toughness of cycloaliphatic epoxy resin 3,4-epoxycyclohexylmethyl-3 ',4 '-epoxycyclohexane carboxylate (ERL-4221) has been improved by using multiwalled carbon nanotubes (MWCNTs) treated by mixed acids. The MWCNT/ERL-4221 composites were characterized by Raman spectroscopy and their mechanical properties were investigated. A significant increase in the tensile strength of the composite from 31.9 to 55.9 MPa was obtained by adding only 0.05 wt % of MWCNTs. And a loading of 0.5 wt % MWCNTs resulted in an optimum tensile strength and cracking energy, 62.0 MPa and 490 N cm, respectively. Investigation on the morphology of fracture surface of the composites by field emission scanning electron microscopy demonstrated the crack pinning-front bowing and bridging mechanisms of toughening. (C) 2008 Wiley Periodicals, Inc.