The change in fracture toughness and its dependence on the content of clay nanoplatelets and adhesion at the interface between clay nanoplatelets and anhydride-cured epoxy matrix are discussed. Three clay nanoplatelets with different chemical modifications were used in this investigation. To fabricate nanocomposites, the clay nanoplatelets were sonicated in acetone for 2 h. The role of the clay nanoplatelets in the mechanical/fracture properties was investigated by transmission electron microscopy (TEM). Bright-field TEM micrographs showed excellent dispersion of clay nanoplatelets in epoxy matrix. Both intercalation and exfoliation of clay nanoplatelets were observed depending on clay modification. Compact tension specimens were used for fracture testing. The fracture toughness increased with increasing clay content. The fracture toughness of clay/epoxy nanocomposites varied with the clay morphology in the epoxy matrix. Different morphologies of the fracture surfaces, highly dependent on the morphology of dispersed clay nanoplatelets, were observed using environmental scanning electron microscopy (ESEM). The fracture toughness was found to be correlated with the fracture surface roughness measured by confocal laser scanning microscopy (CLSM).