Objectives Previous studies have found out that Forsythiae Fructus (FF) extracts have anti-atopic activities by in vitro experiment. In order to understand more about FF extracts' benefit, we subdivided FF extracts depending on systematic fractionation method by using Methylene chloride (MC), Ethyl acetate (EtOAc), n-BuOH and n-hexane (n-Hx). This study is designed to examine the effect of FF fractions on the PMA- ionomycin-induced activation of RBL-2H3 mast cell lines in vitro and on the DNCB-induced activation of NC/Nga mice in vivo. Methods For this study, we examined IL-4, IL-13 production by ELISA analysis, IL-4, IL-13, IL-31, IL-31RA and TNF-${\alpha}$ mRNA expression by real-time PCR and manifestations of AP-1 and MAPKs transcription factors by western blotting in vitro. Through in vitro experiment, we selected FF n-BuOH fraction that seems the best effective in atopic dermatitis then induced it on NC/Nga mice by DNCB. We measured mice's WBC, eosinophil and neutrophil in heart blood, IL-4, IL-5, IFN-${\gamma}$ in the spleenocyte culture supernatant, the absolute cell numbers of CD4+, CD8+, B220+CD23+, CD3+CD69+ and Gr-1+CD11b+ in the PBMCs, ALN and dorsal skin, IL-5, IL-13, IL-31, IL-31RA in the dorsal skin by real-time PCR and the distribution of immune cells by H&E on dorsal skin and ANL and toluidine blue staining on dorsal skin. Results FF n-BuOH fraction suppressed IL-4, IL-13 production and mRNA expression of IL-4, IL-13, IL-31, IL-31RA and TNF-${\alpha}$. Results from the western blot analysis showed that FF n-BuOH fraction reduced the activation of the mast cell specific transduction factors involved in AP-1 by suppressing JNK and ERK phosphorylation. In the gross, atopic dermatitis induced by DNCB in NC/Nga mice were improved by oral administration of FF n-BuOH fraction. Oral FF n-BuOH fraction also decreased the level of IgE in mice's serum and the level of IL-4 and IL-5 in the spleenocyte culture supernatant, cell numbers of CD8+, B220+CD23+ in the PBMCs, CD4+ in the ALN and CD4+, Gr-1+CD11b+ in the dorsal skin and suppressed mRNA expression of IL-5, IL-13, IL-31, IL-31RA in the dorsal skin. Histological examination showed that infiltration levels of immune cells in atopic dermatitis induced NC/Nga mice were improved by FF n-BuOH fraction. Conclusions FF n-BuOH fraction can reduce pruritus by suppressing IL-31, IL-31RA secretion and modulate molecular mediators and immune cells associated with atopic dermatitis induced in NC/Nga mice which may have played a significant role in recovering atopic dermatitis symptoms.