Abstract: Far-infrared absorption spectra of D-, L-, and DL- penicillamines have been measured by terahertz time-domain spectroscopy (THz-TDS) in the frequency range from 0.2 THz to 1.8 THz. The experimental results show large difference among absorption spectra of enantiomers (D- and L- penicillamines) and their racemic compound (DL-penicillamine), which probably originated from the difference of crystal structure and vibration modes of crystal lattice. The study indicates that THz-TDS can be used for discrimination of D-, L-, and DL- penicillamines. The absorption spectra of their mixtures have been fitted by the absorption spectra of D- and L- penicillamines, and we have demonstrated the feasibility of this method to determine quantitatively the mixture ratios of penicillamine enantiomers. These will not only give a new experimental method to distinguish chiral medicine, but also provide a suggestion for further comprehending the interaction of chiral medicine and target biomolecule.