In order to deeply understand the spectroscopic characteristics of L-glutamic acid and L-tyrosine in the terahertz region, terahertz time-domain spectroscopy (THz-TDS) was used to measure the absorption coefficient and refractive index of L-glutamic acid and L-tyrosine in their polycrystalline state at room temperature. A number of well-resolved absorption peaks were observed in the range of frequencies from 0.2 to 2.0 THz, which are interpreted as originating from inter- and intramolecular vibrational modes. The average refractive indices of L-glutamic acid and L-tyrosine are 1.56 and 1.47, respectively. In parallel with the experimental study, density functional theory was applied to obtain structure and vibrational frequencies of the isolated L-glutamic acid and L-tyrosine molecules in gas phase. The vibrational spectra were assigned according to the calculations. Results show that the theoretical calculations are in good agreement with the experimental data and the absorption peaks originate from inter- and intramolecular collective vibrational modes.