To elucidate the morphological origin of the multiple melting endothermic peaks found in DSC scans of poly(tetramethylene succinate), a time-resolved small angle X-ray scattering study utilizing a synchrotron radiation source was conducted. The results suggested that low-temperature endothermic peak can be attributed to the melting of secondary lamellae inserted between the primary lamella stacks, while the middle and high-temperature endothermic peaks were to the melting of primary lamellae. The middle one, however, was associated only with the original ones but the high endothermic peak was with both the original ones and lamellae reorganized through the recrystallization process. A new small angle peak was observed in addition to the original peak during the late stage of melting process. The origin of the new peak was interpreted as a transient phenomenon resulted from the partial melting of primary lamellae prior to the complete melting.