The double melting behavior of poly(butylene terephthalate) (PBT) was studied with differential scanning calorimetry (DSC) and wide-angle X-ray analysis. DSC melting curves of melt-crystallized PBT samples, which we prepared by cooling from the melt (250 degreesC) at various cooling rates, showed two endothermic peaks and an exothermic peak located between these melting peaks. The cooling rate effect on these peaks was investigated. The melt-crystallized PBT sample cooled at 24 K min(-1) was heated at a rate of I K min(-1), and its diffraction patterns were obtained successively at a rate of one pattern per minute with an X-ray measurement system equipped with a position-sensitive proportional counter. The diffraction pattern did not change in the melting process, except for the change in its peak height. This suggests that the double melting behavior does not originate from a change in the crystal structure. The temperature dependence of the diffraction intensity was obtained from the diffraction patterns. With increasing temperature, the intensity decreased gradually in the low-temperature region and then increased distinctly before a steep decrease due to the final melting. In other words, the temperature-dependence curve of the diffraction intensity showed a peak that is interpreted as proof of the recrystallization in the melting process. The peak temperature was 216 degreesC. The temperature-dependence curve of the enthalpy change obtained by the integration of the DSC curve almost coincided with that of the diffraction intensity. The double melting behavior in the heating process of PBT is concluded to originate from the increase of crystallinity, that is, recrystallization.