To obtain the degree of crystallinity of poly(ethylene 2,6-naphthalate) (PEN) sample using IR spectroscopy, we first extracted the pure component spectrum of each crystalline and amorphous phase of PEN via factor analysis using the IR absorbance spectra of unoriented PEN films annealed under various conditions and characterized each absorption peak of the pure component spectrum. And then, in conjunction with least-squares curve-fitting using the pure component spectra, we calculated the degree of crystallinity of the annealed samples. The IR crystallinity exhibited a good correlation with the crystallinity from density measurements. In the case of uniaxially oriented PEN specimens, the IR crystallinity could also be obtained using factor analysis of their form spectra. The polarized IR spectra measured simultaneously with load imposed on the sample during uniaxial stretching enabled us to evaluate changes in the degree of segmental orientation as a function of strain.