The origin of double melting behavior of poly(p-phenylene succinate) (PPSc) was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction. As-polymerized PPSc showed two melting peaks: the low melting (LM) and high melting (HM) peaks at 286 and 311 degrees C, respectively. When PPSc was annealed at 270 degrees C, the LM peak constantly shifted toward higher temperatures and grew in its area with annealing time, and eventually merged into the HM peak located at 308 degrees C. X-ray diffractograms of PPSc annealed at 270 degrees C became sharper with increasing the annealing time while the peak positions did not change. The X-ray diffractograms obtained from the LM and the HM peak exhibited the same diffraction peaks. It was concluded from these results that the double melting behavior of PPSc is due to the distribution of crystals having the same crystal form but differing in size and perfection.