The amoxicillin medicines are widely used in the clinical medicine and ordinary life. The inspection of them is vitally important to the health of the patients and the protection of the legal manufactures. Terahertz is a newly developed electromagnetic radiation with many characteristic properties. In the present paper, three amoxicillin capsules produced by three different companies were studied by the terahertz time-domain spectroscopy technique. The time-domain spectra and the frequency-domain spectra were obtained, and consequently both the absorption coefficient and the refraction index were calculated. The results indicated that the absorption coefficient of the three samples were almost the same, while the refractive indexes of them were different to some extent. It is suggested that the terahertz time-domain spectroscopy is a powerful candidate technique in the quality inspection of penicillin medicines and the crackdowr on the fake drugs.