Background: Shaoyao-Gancao decoction (SGD), a traditional Chinese medicine formula, has been used for the treatment of abdominal pain and dysmenorrhea disease in Asia over long period of time. Its effectiveness has been confi rmed in clinic, but its active constituents remain unclear. Materials and methods: In this paper, a rapid, sensitive and reliable ultra-performance liquid chromatography-electrospray ionization/ quadrupole-time-of-fl ight high-defi nition mass spectrometry (UPLC-ESI-Q-TOF-MS) in positive and negative ion mode were established to characterize the active constituents of SGD in vitro. The analysis was performed on a Waters UPLCTM HSS T3 (2.1 × 100 mm, 1.8 µm) using gradient elution system. Automated MetaboLynxTM technique was employed to screen for the potentially bioactive components in rat plasma after oral administration of SGD. MS/MS fragmentation behavior was proposed for aiding the structural identifi cation of the components. Results: Based on the developed method of fi ngerprint analysis, an injection run of the plasma sample was fi nished in 15.0 min. A total of 12 compounds including 9 prototype components such as gallicacid, albifl orin, liquiritin, pallidifl orin, liquiritigenin, isoLiquiritigenin, formononetin, isolicofl avonol, licoricone, C9H10O3 and 2 metabolites such as liquiritigenin-4'-O-glucuronide, formononetin glucuronide were identifi ed or tentatively characterized. Of note, 3 ingredients were identified from Radix Paeoniae Alba, and 9 were from Radix Glycyrrhizae. Conclusion: The compounds found in dosed plasma could be the effective substances of SGT for treating dysmenorrheal, and may provide important experimental data for further pharmacological and clinical research of SGD. Furthermore, this work has demonstrated that the feasibility of the UPLC-ESI-Q-TOF-MS for rapid and reliable characterization of identifi cation and structural elucidation of the chemical constituents and their metabolites from herbal medicines.