The orthorhombic-to-hexagonal phase transition for constrained ultradrawn polyethylene fibers has been investigated by solid-state C-13 NMR spectroscopy. In the CP/MAS C-13 NMR spectra, the resonance line assignable to CH2 carbons in the hexagonal phase appears above 147 degrees C upfield compared to the line in the orthorhombic phase. The calculation of the C-13 chemical shift has revealed that 7% gauche defects are included in the hexagonal phase. In the CP/DD C-13 NMR spectra measured by setting the orientation axis perpendicular to the static magnetic field, two resonance lines assignable to sigma(11) and sigma(22) for the CH2 carbons that appear in the orthorhombic phase merge to a single resonance line at an upfield position compared to the average of sigma(11) and sigma(22) in the hexagonal phase. On the other hand, when the orientation axis is set parallel to the static magnetic field, no remarkable change is observed in the hexagonal phase for the resonance line appearing at sigma(33) These experimental results indicate that gauche defects such as kinks should be introduced at random along each molecular chain, and independent jump rotations occur around the molecular chain axis for the sequences between the gauche defects in the hexagonal phase. It has been also found that the 180 degrees jump rotation around the molecular chain axis is induced in the orthorhombic phase at temperatures near the orthorhombic-to-hexagonal phase transition.