Abstract: Sulfamethoxazole (SMX), one of the well-known effective and tolerable antibacterial agents, is widely used to treat urinary tract infections. Due to the variations in crystallinity structure, SMX exist in two different polymorphic forms I and II. These polymorphs have the same chemical composition but unique physio-chemical properties which will influence the corresponding different stability, solubility, dissolution rate and also other performance characteristics. Terahertz time-domain spectroscopy(THz-TDS) becomes a promising analytical technique relevant to the pharmaceutical and biological fields. THz-TDS measurement is able to characterize and distinguish the different polymorphs according to the rich information yielded, due to the sensitivity of THz absorption in this low-frequency region to the structures over length scales greater than intramolecular bond distances. In this study, absorption spectra in the terahertz region between 6 and 50 cm-1 (0.2-1.5 THz) were measured for SMX pharmaceutical molecule with different polymorphic forms (forms Ⅰ and Ⅱ, and also raw material) using THz -TDS technique at room temperature. The temperature-dependent THz-TDS of the raw SMX material was also investigated between 95 K and 296 K. Different absorption features were observed for these two model polymorphs. The observed THz absorption bands are strikingly sensitive to the change of subtle conformational structures existed within such crystal molecules with different polymorphism. The results show that the THz -TDS technique is a promising method in solid-state analytical tools to distinguish or differentiate such compounds with different polymorphs in pharmaceutical and biological fields.