The THz absorption within the range of 0.2—2.8 THz（6.7—93.2 cm－1）and Raman scattering spectra within the range of 10—4000 cm－1 of polycrystalline L-threonine have been examined with terahertz time-domain spectroscopy （THz-TDS） and Raman spectroscopy. Infrared spectrum between 400 and 4000 cm－1 were also recorded. We assigned the polar and the non-polar vibrational modes of the polycrystalline molecule in the range of both 6.7—93.2 cm-1 and 400—4000 cm-1 based on the theory of space group and the comparison of Raman and absorption spectra. Normal frequency vibrational modes were calculated for the single crystal form using the B3LYP method of density functional theory （DFT） and 6-311+G** basis set. All the experimental THz absorption peaks and low frequency Raman scattering peaks of polycrystalline L-threonine were assigned.