In situ Fourier transform infrared (FTIR) measurements were carried out to elucidate conformation changes occurring during the isothermal melt crystallization of poly( ethylene-2,6-naphthalate) (PEN). Based on the band assignments for the components of the amorphous, alpha-crystal form, and beta-crystal form of PEN in film samples, the in situ data was analyzed in terms of the amorphous-and crystal-trans conformations. It was observed at a higher isothermal crystallization temperature that the formation of amorphous-trans conformations precedes the growth of crystals.