Preordering behavior of poly(trimethylene terephthalate) (PTT) upon cold crystallization was revealed via in situ simultaneous small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC) as well as time-resolved Fourier transform infrared (FTIR) spectroscopy. In the induction stage prior to the appearance of WAXS crystalline peaks, changes in the low-q (0.005-0.03 angstrom(-1)) range of SAXS profiles suggested the formation of mesomorphic domains (with a linear dimension similar to 18 nm). Parallel DSC and FTIR results indicated respectively that the preordering involved a trace amount of exothermic heat prior to discernible trans-to-gauche conformation change. Subsequently observed were the growth of these mesomorphic domains to a size approximate to 50 nm and the accompanying emergence of nanocrystalline grains (of dimensions of several nanometers) via a more localized nucleation-and-growth route. The final ripened structure hence comprised a percolated network of mesomorphic domains filled with dispersed nanocrystallites. Hierarchical evolutions of the large mesomorphic domains and the small nanocrystallites with crystallization time were correlated with WAXS-determined crystallinity development and DSC-revealed endothermic events. Transitions from the amorphous to mesomorphic domains and the mesomorphic to crystalline domains are illustrated quantitatively. The formation of large mesomorphic domains in the induction stage of crystallization and their influences on the subsequent crystallization are discussed in terms of liquid liquid phase separation and gel formation.