Three prolyl endopeptidase inhibitors were isolated and identified as luteolin, quercetin and ${eta}$-sitosterol-3-O-${eta}$-D-glucopyranoside with $IC_{50}$ of 0.17, 0.19 and 27.5 ppm, respectively. The inhibition of two flavonoids were non-competitive with substrate. Twenty authentic flavonoids were tested in order to investigate structure-activity relationship. No significant relationship was found in them, however, catechol moiety of B-ring and 7-OH group in flavonoid skeleton were seemed to be responsible for the stronger activity.