In this work, the structure of poly(trimethylene terepthalate) (PTT)/monolayer nano-mica (MNM) nanocomposites are investigated by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS) and scanning electron microscope (SEM). In the PTT nanocomposites, the crystallization induces the segregation of MNM in which three morphologies, including interlamellar, interfibrillar, and inter-spherulitic segregations, are observed with changing the MNM content. Avrami analysis of isothermal crystallization demonstrates that MINIM enhances the bulk crystallization rate in the nanocomposites. Moreover, the non-integral values of Avrami exponent n between 2 and 4 with increasing crystallization temperature indicate the mixed growth and nucleation mechanisms. The analysis of secondary nucleation theory for neat PTT and the PTT nanocomposites exhibit the same regime transition of crystallization behaviour in which the classical transition temperatures of regime I to II and regime II to III take place at 488 K and 468 K, respectively. The growth rate of spherulites of the PTT nanocomposites is twofold larger than that of neat FIT in regime III, implying that MNM plays an effective role as a nucleating agent, since the addition of MINIM enormously reduces the activation energy of nucleation, folding surface free energy and average work of chain folding for PTT nucleation. However, experimental results show that the MNM content below 1 wt% is the most effective for nucleation of PTT crystallization. (C) 2009 Elsevier B.V. All rights reserved.