The isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) have been investigated using differential scanning calorimetry (DSC) and polarized light microscopy (PLM). Enthalpy data of exotherm from isothermal crystallization were analyzed using the Avrami theory. The average value of the Avrami exponent, n, is about 2.8. From the melt, PTT crystallizes according to a spherulite morphology. The spherulite growth rate and the overall crystallization rate depend on crystallization temperature. The increase in the spherulitic radius was examined by polarized light microscopy. Using values of transport parameters common to many polymers (U* ฯญ 1500 cal/mol, T ฯฑ ฯญ T g ฯช 30 ยฐC) together with experimentally determined values of T m 0 (248 ยฐC) and T g (44 ยฐC), the nucleation parameter, k g , for PTT was determined. On the basis of secondary nucleation analyses, a transition between regimes III and II was found in the vicinity of 194 ยฐC (โฌT ะฅ 54 K). The ratio of k g of these two regimes is 2.1, which is very close to 2.0 as predicted by the Lauritzen-Hoffman theory. The lateral surface-free energy, ฯญ 10.89 erg/cm 2 and the fold surface-free energy, e ฯญ 56.64 erg/cm 2 were determined. The latter leads to a work of chain-folding, q ฯญ 4.80 kcal/mol folds, which is comparable to PET and PBT previously reported.