The isothermal melt crystallization of poly(trimethylene terephthalate) (PTT) has been investigated using differential scanning calorimetry (DSC), optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. Triple-melting endotherms were dominant at crystallization temperatures (T-c) below 195 degrees C, while double-melting endotherms were dominant at T-c above 195 degrees C. These multiple-melting behaviors were attributed to melting and recrystallization as well as thermal stability of the crystallites. The difference in crystallinity observed between melt-crystallized PTT films before and after cooling to room temperature was attributed to lamellar thickening. The kinetics of isothermal melt crystallization of PTT was analyzed using an Avrami equation, and the rate constant (k) and t(1/2) increased with increasing T-c. An Avrami exponent n = 2-3 was obtained for the PTT melt crystallized from 150 to 205 degrees C, suggesting two-dimensional to three-dimensional growth with heterogeneous nucleation and was confirmed by polarized optical microscopy. The bands at 1358 and 976 cm(-1) associated with gauche and trans conformations of -OCH2CH2CH2O- were used to determine gauche and trans conformations assuming a two-phase conformational model. The crystalline and the amorphous gauche conformations were separated by combining them with DSC crystallinity. It was shown that the crystalline gauche conformation increased at the expense of the amorphous trans conformation. No significant change in the amorphous gauche conformation was found. On the other hand, the crystalline and amorphous gauche conformations were found to increase with the annealing temperature (T-a) at the expense of the trans conformations during the annealing of PTT. The difference between melt crystallization and annealing was also discussed.