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Terahertz time-domain spectroscopy (THz-TDS) is a technique based on femtosecond,short pulse lasers,which enables to investigate both the frequency-dependent absorption coefficients and refractive indices of material in the far-infrared region very precisely.Now,this technique is widely applied to various materials and molecules including s and biomolecules to study intermolecular interactions and low-frequency dynamics.In this work we have measured frequency-dependent absorption coefficients and refractive indices of amino-acids (glycine and L-alanine) and polypeptides (polyglycine and poly-L-alanine) in the wavenumber region from 7 cm-1 to 55 cm-1 by THz-TDS.A vibrational band was observed at 45.5 cm-1 for polyglycine,which was assigned as an interchain mode.