The intrinsic lateral sonic moduli of the crystalline and amorphous regions of PEN (ca. 6.161 and 2.357 GPa, respectively) were obtained from sonic moduli and crystallinities of unoriented poly(ethylene-2,6-naphthalate) (PEN) specimens which were annealed under different conditions. Using the obtained intrinsic lateral sonic moduli of the crystalline and amorphous regions, the degree of orientation of the amorphous region of uniaxially drawn PEN filaments could also be obtained from the crystallinity, sonic modulus, and the degree of crystalline orientation estimated from X-ray azimuthal scan data. Thus, the intrinsic birefringences of the perfectly oriented crystalline and amorphous regions of PEN, ca. 0.463 and 0.504, respectively, could be obtained from the combination of birefringence, crystallinity, and the degree of orientation of the crystalline and amorphous regions.