In order to explore the feasibility of medicine inspection by terahertz time-domain spectroscopy technique, 3 amoxicillin capsules from 3 different companies were studied by a terahertz time-domain spectroscopy system built by the authors. In spite of certain differences compared with the results from literatures in nitrogen condition, all 3 amoxicillin samples here presented obvious characteristic absorptions. The positions and the amplitudes of the absorption peaks and the refraction indexes of 3 samples were different to one another to some extent. The experiment results confirmed the application prospects of on-line inspection and quality control of penicillin medicines by the terahertz time-domain spectroscopy technique.