During the cold crystallization of poly(trimethylene terephthalate), FIT, multistage preordering activities associated with hierarchically structural evolution and origin of mesomorphic domains were investigated with time-resolved Fourier transform infrared (FTIR), photoluminescence (PL) and dielectric spectra (DS), and simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS). The observed fluorescent emission at 390 nm and an absorption band at 874 cm(-1) associated with the CH out-of-plane bending mode of the phenyl rings indicate that the ground-state dimers resulting from the phenyl rings of PIT chains via pi-pi interactions are formed in supercooled amorphous liquids. The time dependences of the increase of the fluorescent intensities, the wavenumber shift from 874 to 872 cm(-1), the conformational transformation of the trimethylene glycol units from trans to gauche, the dynamic alpha-to-alpha' transition of the dielectric loss and the structural parameters reveal that the dimer aggregation accompanying a locally nematic-like orientation induced large phase-separated mesomorphic domains (over several tens of nanometers); nucleations of nanograin type (over several nm) subsequently formed within the mesomorphic domains after the conformational transformation form trans to gauche for the cold crystallization at 50 degrees C. Herein, we propose a preordering mechanism of cold crystallization in which the pi-pi interactions among the dimers serve as ordering bonding forces to drive intermolecular cooperativity before intramolecular changes of conformation, and the dimer aggregation triggers the phase-separated mesomorphic domains before the growth of crystalline nanograins (non-lamellar crystals). As distinct from the hierarchical structure of nanograins within mesomorphic domains, the formation of lamellar crystallites within spherulites without undergoing the dimer aggregation were observed form the melt crystallization at 200 degrees C. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.