Nylon-66 nanocomposites were prepared by melt-compounding nylon-66 with organically modified montmorillonite (MMT). The organic MMT layers were exfoliated in a nylon-66 matrix as confirmed by wide-angle X-ray diffraction (WAXD) and transmission electron microscopy. The presence of MMT layers increased the crystallization temperature of nylon-66 because of the heterogeneous nucleation of MMT. Multiple melting behavior was observed in the nylon-66/MMT nanocomposites, and the MMT layers induced the formation of form II spherulites of nylon-66. The crystallite sizes L-100 and L-010 of nylon-66, determined by WAXD, decreased with an increasing MMT content. High-temperature WAXD was performed to determine the Brill transition in the nylon-66/MMT nanocomposites. Polarized optical microscopy demonstrated that the dimension of nylon-66 spherulites decreased because of the effect of the MMT layers. (C) 2003 Wiley Periodicals, Inc.