Upon the rennin-angiotensin system, angiotensin Ⅰ-converting enzyme (ACE) inhibitors play critical roles in alleviating and suppressing hypertension. The study was performed to isolate and purify an angiotensinⅠ-converting enzyme inhibitory peptide from papain digests of Spirulina platensis by ultra-filtration, gel filtration chromatography and reverse-phase high-performance liquid chromatography. The purified peptide was identified by matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and amino acid sequencing. Furthermore, the inhibition pattern of the peptide was also investigated and the stability was evaluated under simulated gastrointestinal condition. The results demonstrated that the digests with molecular mass ranging from 0 to 3 000 ku had the most potent ACE inhibitory activity with an IC₅₀ value of (1.03 ± 0.04) g/L. An ACE inhibitory peptide with an IC₅₀ value of (0.009 4 ± 0.000 2) g/L which was equivalent to (27.36 ± 0.14) μmol/L was obtained from that fraction of digests, and was identified as Val-Glu-Pro. The Lineweaver-Burk plot and the Dixon plot indicated the ACE inhibitory peptide was a non-competitive inhibitor with a K_i value of (23.59 ± 0.54) μmol/L. In vitro stability assay showed that the peptide could keep its inhibitory activity well after incubation with gastrointestinal proteases including pepsin, chymotrypsin, and trypsin, suggesting the ACE inhibitory peptide from Spirulina platensis be of great prospects as an ingredient of functional foods or pharmaceuticals in prevention and treatment of hypertension.