The root of Paeonia lactiflora PALL (PL, Family Paeoniaceae) has been used frequently as an antipyretic and anti-inflammatory agent in traditional medicines of Korea, China and Japan. To evaluate antiallergic effect of PL, we isolated its main constituents, paeoniflorin and paeonol, and evaluated in vivo their inhibitory effects against passive cutaenous anaphylaxis (PCA) reaction induced by IgE-antigen complex and scratching behaviors induced by compound 48/80. PL, paeoniflorin and paeonol potently inhibited PCA reaction and scratching behaviors in mice. Paeoniflorin exhibited the most potent inhibition against scratching behaviors and the acetic acid-induced writhing syndrome in mice. Paeonol most potently inhibited PCA reaction and mast cells degranulation. These findings suggest that PL can improve IgE-induced anaphylaxis and scratching behaviors, and may be due to the effect of its constituents, paeoniflorin and paeonol.