A high-tension annealing (HTA) method has been applied to zone-annealed poly(ethylene-2,6-naphthalate) (PEN) fibers in order to further improve their mechanical properties. The HTA treatment was carried out under an applied tension of 428 MPa at a treating temperature of 175 degrees C. The applied tension was close to the tensile strength at 175 degrees C. The resulting HTA fiber had a birefringence of 0.492 and degree of crystallinity of 57%. Wide-angle X-ray diffraction (WAXD) photographs of the HTA fibers showed three reflections (010, 100, and (1) over bar 10) attributed to an alpha form crystal, but no (020) reflection attributed to a beta form was observed in the equator. The tensile modulus and tensile strength increased with processing, and the HTA fiber had a maximum modulus of 33 GPa, a tensile strength of 1.1 GPa, and a storage modulus of 33 GPa at 25 degrees C.