Epoxy/MMT composite material and a crosslinking method for epoxy/MMT composite material with second order nonlinear optical properties are introduced. Chromophore-containing intercalating agents are applied to modify montmorillonites (MMTs) to form organoclays by an ion-exchange process, wherein the chrmophores are neatly packed on exfoliated epoxy/organoclay nanocomposites. As a result, optical nonlinearity, i.e. the Pockels effect is observed for the nanocomposites without resorting to the poling process due to self-assembly process. Furthermore, a series of epoxy/MMT nanocomposites comprising thermally reversible furan-norbornene Diels-Alder adducts are prepared to establish a crosslinking feature. Self-alignment behavior, electro-optical (EO) coefficient and temporal stability of the epoxy/MMT nanocomposites are improved by the Diels-Alders crosslinking.