The quantitative analysis of amino acids by terahertz (THz) time-domain absorption spectroscopy is demonstrated. The optical densities of the amino acids were found to be linearly proportional to the concentration. The molar absorption coefficients of L-glutamic acid (L-Glu), L-glutamic acid sodium salt (Na-L-Glu), L-glutamic acid hydrochloric salt (HCl-L-Glu), L-cysteine (L-Cys), and L-histidine (L-His) were calculated by averaging the THz spectra of the amino acids at several different concentrations in approximately the 0.2-1.0 mol L(-1) range. The concentrations of L-Glu, L-Cys, and L-His mixed samples were successfully calculated with errors of less than 11% and 20% when their concentrations were higher than 0.45 and 0.22 mol L(-1), respectively, by using the obtained molar absorption coefficient.