Radix Rehmanniae (RR) is the root tuber of Rehmannia glutinosa Libosch. Herein, the methanol extracts of dried RR (DRR) and processed RR (PRR) were partitioned to obtain ethyl acetate, aqueous, and n-butanol layers. The angiotensin-I converting enzyme (ACE) inhibition test indicated that the ethyl acetate extracts of DRR (DRRE) and PRR (PRRE) show better inhibitory activity. Therefore, changes in blood pressure were tested over 24 h in spontaneously hypertensive rats, with DRR showing good anti-hypertensive activity. DRRE was further subjected to column chromatography; 28 fractions were separated and tested for ACE inhibition. Ultimately, six compounds were identified by spectral analysis, and literature comparison. Specifically, ursolic acid and oleanolic acid showed better ACE inhibition than the other compounds. This study confirmed that DRR has anti-hypertensive activity. In future, DRR's potential as a health food should be further assessed.