An angiotensin converting enzyme (ACE) inhibitory substance was isolated and purified from Lycium chinense Miller. A crude water extract of Lycium chinense Miller was prepared by adding it to water shaking at $25^{circ}C$ for 1 hr, followed by centrifugation at 8000 ${ imes}$ g for 30 min. The crude extract was then filtered using YM-3 and YM-1 membranes. An ACE inhibitor was isolated using consecutive chromatographic methods including: ion exchange chromatography, gel permeation chromatography, and FPLC. The inhibitor was identified to have a molecular mass of 862 daltons by mass spectrometry.