Background: Fifteen medicinal plants used in traditional Thai herbal medicine (HM) for hypertension treatment were previously tested for their inhibitory activity against an angiotensin-converting enzyme (ACE). The ethyl acetate extract from one of these medicinal plants, Senna garrettiana, possessed satisfactory ACE inhibitory (ACEi) activity. Objective: For this study, S. garrettiana extract was further subjected to an isolation process to uncover the active compounds with ACEi activity. Materials and Methods: The ethyl acetate fraction was subjected to isolate and purify by column chromatography accompany with the ACEi assay. The structures of all compounds were elucidated using nuclear magnetic resonance spectroscopy. Results: It was found that S. garrettiana extract possessed two active compounds, piceatannol and betulinic acid, which both had good ACEi activity displaying IC50values of 8.44 μM and 26.77 μM, respectively. Conclusions: These findings show that both compounds can be used as markers for quality control of any standardized HM for hypertension treatment and not just for traditional Thai herbal remedies. Moreover, to the best of our knowledge, this is the first report investigating both compounds for their ACEi activity.