In this study, the fatigue crack propagation (FCP) of epoxy clay nanocomposites is investigated. Especially effect of size, aspect ratio and tactoid shear - stiffness of clay nanoplatelets are analyzed in detail. To achieve a systematical approach, two highly pure synthetic fluorohectorites having large lateral extension (averagely 3.8 mu m) with different tactoid shear stiffness states were used and compared to natural Montmorillonite (MMT) with significantly smaller tactoid lateral size (400 nm). A novel nanocomposite preparation method combining solvent casting with 3-roll milling was applied to achieve excellent dispersion of nano-additives. The FCP behavior is improved by incorporation of all types of layered silicates, however highest FCP resistance was observed by shear stiff fluorohectorites with large lateral extension. Responsible toughening mechanisms are mainly depending on the lateral extention of nanoclays. This can be explained by SEM micrographs of fracture surfaces and relating the particle size with dynamic crack tip opening displacements.