By manipulating processing conditions, three levels of exfoliation of synthetic a-zirconium phosphate (a-ZrP)nanoplatelets in epoxy matrices have been achieved. Transmission electron microscopy and wide angle X-ray diffraction were utilized to confirm the three different levels of exfoliation of nanoplatelets in epoxy/a-ZrP nanocomposites. As expected, it was found that modulus and strength of the nanocomposite are affected by how well the nanoplatelets disperse. It was also found that the operative fracture mechanisms depend strongly on the state of the nanoplatelets dispersion. The crack deflection mechanism, which leads to a tortuous path crack growth, was only observed for poorly dispersed nanocomposites. Delamination of intercalated nanoplatelets and crack deflection were observed in a moderately dispersed system. In the case of fully exfoliated system, the crack only propagated in a straight fashion, which indicates that the fully exfoliated individual nanoplatelet can not affect the propagation of crack at all. The implication of the present findings for structural applications of polymer nanocomposites is discussed.