The mechanical properties of epoxy/clay nanocomposites were investigated in relation to the particle size and shear stiffness state of the dispersed nanoplatelets. The fracture toughness and the underlying toughening mechanisms were thoroughly discussed in detail. For this study, a highly pure synthetic fluorohectorite with large lateral extensions (a parts per thousand 3.8 A mu m) was used and compared to natural montmorillonite characterized by significantly smaller lateral extensions (a parts per thousand 400 nm). Moreover, for the synthetic fluorohectorite, the subtle balance between layer charge density and the hydration enthalpy of interlayer cations allows for switching between a shear-labile and shear-stiff state, something impossible for the natural material. To ensure optimum dispersion, solution blending was followed by three roll milling for nanocomposite preparation. The addition of all three types of clay used in this study provoked a decrease in glass transition temperature, which indicated a moderate interfacial strength. The maximum increase in fracture toughness and strain energy release rate was observed for the nanocomposites prepared with the large and shear-stiff fluorohectorites at a particle content as low as 2.2 vol%. Morphological investigations by scanning electron microscopy of the fracture surfaces revealed the contribution of several micro-mechanical toughening mechanisms. In contrast to the small natural montmorillonite, the large synthetic nanoplatelets promoted additional energy dissipating mechanisms such as crack deflection and crack pinning leading to an enhanced fracture toughness. These observations are discussed in details using fracture mechanical approaches.