Prolyl endopeptidase(PEP) ［EC 3.4.21.26］ is a new class of serine protease, which is capable of hydrolyzing a substrate on the carboxyl site of proline residue located internally in a peptide. It can hydrolyse several peptide hormones and neuotransmitters, so abnormal raise and decrease of PEP activity would result in diseases related to memory and cognition. Specific inhibitor for example JTP-4819 shows a good pharmaceutical effect in reversing scopolamine-induced amnesia in rats. The analysis of crystral structure of PEP from porcine muscle has greatly promoted the research on PEP.