Although the effects of the incorporation of nanoparticles in polymer matrices are deeply studied in the literature, the mechanisms that promote higher properties are not well reported. In this paper, the reinforcement and toughening mechanisms that act on nanoclay/epoxy nanocomposites were studied. For this purpose, two types of nanoclays were used (sodium nanoclay and organophilic nanoclay) in three different concentrations. We propose the reinforcement effectiveness concept, which provides a way to assess the reinforcement mechanism without the use of TEM images. The fracture toughness test and the analysis of the fracture surface provided evidence that for nanoclay/epoxy nanocomposites, the dominant toughening mechanism is strongly dependent on the dispersion state, with a transition from the crack pinning to the crack deflection mechanism. (C) 2015 Elsevier B.V. All rights reserved.