Low-frequency vibrational spectroscopy of polymers have been studied in far-infrared and THz frequency regions, and the relationship between THz spectral features and polymer structures and properties has been studied. In our previous studies, the THz spectra of polyesters and polyamides were studied. In order to assign the spectra, the vibrational dipole moments were determined using polarized THz spectroscopy on stretched samples, and the experimental THz spectra was compared with low-frequency Raman spectra in order to uncover the origins of the vibrational modes. Additionally, quantum mechanical calculations were applied to obtain low-frequency vibrational modes. In this regard, the coupling of THz spectral measurements with ab initio simulations provides unparalleled insight into the forces and dynamics that drive the properties of materials. Advances in computational resources have permitted increasingly large calculations to be performed, which has recently proven to be valuable for a wide-range of applications, including phase-transformation phenomena, organic semiconducting efficiency, and pharmaceutical stability, to name a few. In those studies, the structural information obtained by THz spectroscopy is in good agreement with those obtained by conventional methods, such as differential scanning calorimetry (DSC), NMR, and X-ray diffraction measurements. Moreover, THz spectroscopy shows great advantages for in situ spectroscopy, such as time-lapse monitoring of the isothermal crystallization or the imaging of the spatial distribution of higher order structures in the samples. Another advantage of THz spectroscopy is the high sensitivity to water molecules. Water molecules show intense absorption in THz frequencies due to their large permanent dipole moment. Therefore, even small amounts of water contamination in the samples can be detected by THz spectroscopy. For macromolecules, the THz spectra of adsorbed water has long been studied, along with the spectral change with the hydration has been studied. In most of those studies, measurements have been performed using THz-TDS or dielectric spectroscopy, and only acquired below 3 THz, where only broad spectral changes were observed. On the other hand, THz spectra above 4 THz provides significantly more detailed information about water dynamics, because in those frequencies, intermolecular vibrational motion (S band at 5.2 THz) and libration motion (L1 and L2 bands at 12 and 20 THz) are observed. In our previous paper, THz spectra of hydrated polymer films were measured from 1-9 THz, and dynamics of bound water were discussed by the intensity ratio between vibrational and librational modes. In this study, the hydration of nylon-6 films was observed with THz spectroscopy. Nylon-6 is a crystalline polymer used in various industrial applications. The structure of nylon-6 consists of amide groups that form a NHCO hydrogen bonding network between polymer chains, which makes nylon-6 hydrophilic. In 1960's, Puffr et al. proposed the structure model of water in polyamides as three types of water molecules. The first type forms double hydrogen bonds with the CO groups, the second forms hydrogen bonds with both the CO and NH groups, and the third exhibits no hydrogen bonds with nylon. After that, the dynamics and structure of adsorbed water in nylon-6 has been studied using neutron and X-ray scattering, NMR, infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations, but never with lowfrequency vibrational spectroscopy, which is much more sensitive to intermolecular interactions than other techniques.