Technical Abstract: This research was to develop an antihypertensive peptide, an efficient angiotensin converting enzyme (ACE) inhibitor (ACEI), from Porphyra yezoensis. Seven commercial enzymes were screened and then enzymatic hydrolysis conditions were optimised. The results showed that alcalase was the most effective for hydrolysis and its optimum conditions for achieving the highest antihypertensive activity of peptide were 1.5% substrate, 5% alcalase, pH 9.0, and temperature of 50 _C at a hydrolysis time of 60 min. The antihypertensive peptide produced under the optimum conditions had a high ACE inhibition rate of 55.0% and a low IC50 value of 1.6 g/l and remained at high stability at temperatures of 4, 25, and 37 _C, pH values of 2.0 and 8.0, and after pepsin and trypsin treatments. Major proteins from P. yezoensis were glutelin, albumin, and gliadin. The albumin and glutelin had higher hydrolysis rates than the gliadin, but the IC50 value of glutelin was the lowest, which indicated that the antihypertensive peptide from glutelin was more functional.