Double melting behavior of poly(trimethylene terephthalate) (PTT) was studied in detail by means of differential scanning calorimetry (DSC) and optical microscopy. The results indicate that the low-temperature melting peak of PTT at ca. 218 degrees C for the samples crystallized isothermally at 203 degrees C is associated with the melting of crystals produced by secondary crystallization, while the high-temperature melting peak of it at about 227 degrees C is related to the melting of the crystals produced by primary crystallization. The results further demonstrate that the PTT crystals growing non-isothermally during cooling process are thermodynamically unstable and can undergo structure reorganization during the DSC heating scan. The reorganized crystals melt at temperature higher than the crystals produced by secondary crystallization at 203 degrees C. Consequently, for the non-fully crystallized samples, the crystals grown during cooling also exhibit contribution to the high-temperature melting peak.