Two new guaiane-type sesquiterpene (1, 2), 2 new phenylpropanoid derivatives (3, 4), and 5-oxo-11-hydroxy-8(Z)-undecenoic acid 11-O-glucoside (5), together with 17 known compounds have been isolated from the whole plants of Youngia japonica (L.) Dc., which have been known to be used as folk medicines to treat people suffering from atopy. The guaiane-type sesquiterpene, grosheimin (17) exhibited strong antiallergic and antioxidant activities.