The germination of Lactuca sativa seeds was significantly inhibited by the water extract of the fresh leaves of Pulsatilla koreana N. including abundant ranunculin. Germination inhibitory activity increased in a dose-dependantly. Protoanemonin, produced from ranunculin by enzymatic action during maceration process of leaves, was proved to be the active principle with inhibitory activity was above 90% at concentration of 1 mg/ml.