Angiotensin-converting enzyme (ACE) was purified from human plasma by affinity chromatography. Effect of ethyl acetate, butanol and water extracts of Juniperus excelsa Bieb. (J. excelsa) fruits on purified ACE activity was investigated. ACE was purified 3659-fold with a specific activity of 1350 EU/mg protein from human plasma. The purity and molecular weight of ACE were determined by SDS-PAGE, and two bands 60kDa and 70kDa are seen on the gel. Ethyl acetate extract of J. excelsa fruits showed an activation effect on human plasma ACE activity. Butanol and water extracts of J. excelsa fruits showed an inhibition effect on ACE activity. IC50 values for butanol and water extracts of J. excelsa fruits were calculated to be 2.858mg/mL and 5.790mg/mL, respectively. Lisinopril was used to be reference inhibitor. Type of inhibition for all inhibitors was designated as non-competitive. IC50 value and K-i constant for lisinopril were determined to be 0.781nM and 0.662nM, respectively. These results show that butanol and water extracts of J. excelsa plant may have an ACE inhibitor potency.