The bisphenol-F type flexible epoxy resin, having a good flexibility, was combined with an organo- and slurry-clay. The clay dispersions in the obtained epoxy/clay systems are significantly different depending on the type of clay. Particularly, the epoxy/slurry-clay system showed a high clay dispersibility into the epoxy matrix and was transparent in spite of the addition of 10 wt % clay. This result means that the swelling of the clay with formamide is effective for the expansion of the basal spacing of the clay. The slurry-clay nanocomposite (clay content: 5 wt %) showed a 4 times higher fracture energy than the neat epoxy system in the tensile test, though the organo-clay system (clay content: 5 wt %) was 1.5 times higher. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2256-2263, 2009