Poly-L-lysine is known to have three different secondary structures depending on solvent conditions because of its flexible nature. In previous work (Kambara et al., Phys Chem Chem Phys 2008, 10, 5042-5044), we observed two different types of structural changes in poly-L-lysine. In the present study, we investigated the low-frequency spectrum of poly-L-lysine with a beta-sheet structure in the solid state by terahertz time-domain spectroscopy. On the basis of this spectroscopic analysis, we found that the low-frequency dynamics differed from those of other polypeptides. Furthermore, we performed powder X-ray diffraction measurement on poly-L-lysine, which was found to be highly amorphous compared with other polypeptides.