Information on the crystalline structure and the properties of poly(butylene-2,6-naphthalene dicarboxylate) (PBN) has not been well reported until now, but it is known that there are two different crystal modifications in PEN, as follows: one is formed in isotropic samples by annealing (alpha form); another appears by annealing with tension (beta form). The relation between the crystal modifications and the kinetics of isothermal crystallization for PEN was investigated using in-situ Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (WAXD). The melting behavior of each crystalline form was also studied by means of FTIR and differential scanning calorimetry (DSC) measurements. From the analysis of the melt-crystallized PEN specimens, the two crystalline forms coexisted in the isotropic samples melt-crystallized at 230 degrees C, but only the alpha crystal modification was observed in the films annealed at lower temperatures. In addition, it was revealed that, at 230 degrees C, the beta modification was formed only in the primary crystallization process.