Absorption spectra of polycrystalline L-, D-, and DL-alanine have been measured by the terahertz time domain spectroscopy (THz-TDS) in the frequency range from 10 to 90 cm-1 at room temperature. We observed several absorption bands, which have the large difference between enantiomers (L- and D-alanine) and the racemic compound (DL-alanine) in their peak frequencies. This obvious difference shows that the THz absorption bands are strikingly sensitive to the crystal structures. This result indicates that the THz-TDS can be used for distinguishing between the enantiomers and the racemic compound. We propose and demonstrate a method to determine the enantiometric composition of amino acids from the THz absorption spectra.