Crystallization of Poly (trimethylene terephthalate) (PTT) by annealing was examined using density measurement, differential scanning calorimetry (DSC) and far-infrared spectroscopy (FIR). Crystallinity, measured by density, increased slowly up to the Ta of 185oC and increases rapidly once Ta is exceeded 185oC. It was found that thermally induced crystallization is mainly temperature dependent at above Ta =185oC and temperature and time dependent below Ta = 60oC. Two melting transitions, Tm1 and Tm2 were observed for those samples annealed at above 120oC. No significant change in Tm1 was observed as a function of Ta while Tm2 showed strong dependency on Ta. Digital subtraction of the amorphous contribution from the semicrystalline FIR spectra provided characteristic spectra of amorphous and crystalline PTT. The bands at 373, 282 and 92 cm-1 were assigned to the crystalline phase, while the bands at 525, 406 and 351 cm-1 were attributed to the amorphous phase. It was shown that FIR spectroscopy can be used as a means to estimate the degree of crystallinity of PTT. The band ratio of 373 and 501 cm-1 was plotted against crystallinity measured by density and reasonably good correlation was obtained.