Objectives Forsythiae Fructus treatment has been used for inflammatory and allergic diseases in Korean Medicine. Nevertheless, the mechanism of action and the cellular targets are not understood well. The pathogenesis of allergic diseases are associated with Th2 cytokines such as IL-13, MIP-$1{alpha}$, IL-13, IL-5, GM-CSF, IL-4, TNF-${alpha}$ and IL-6, which are secreted by the mast cells. This study was conducted to investigate the effects of Forsythiae Fructus extracts (FF) on Th2 cytokines expression and signal transduction in MC/9 mast cells. Methods In the study, MC/9 mast cells were stimulated with DNP-IgE for 24 hours and then treated separately with CsA $10{mu}g/m{ell}$ and varying doses of FF for one hour. MC/9 mast cells stimulated with DNP-IgE was the control group, a treatment with CsA was the positive control group and a treatment with varying doses FF was the experimental groups. The mRNA levels of IL-13, IL-5, GM-CSF, IL-4, TNF-${alpha}$, IL-6 were analyzed with Real-time PCR. The levels of IL-13, MIP-$1{alpha}$ were measured using enzyme-linked immunosorbent assays(ELISA). NFAT, AP-1 and NF-${kappa}B$ p65 were examined by Western blot analysis. Results 1. FF were observed to suppress the mRNA expression of IL-13, IL-5, GM-CSF, IL-4, TNF-${alpha}$, IL-6 in comparison to DNP-IgE control group. 2. FF also has inhibited the IL-13, MIP-$1{alpha}$ production significantly in comparison to DNP-IgE control group. 3. Western blot analysis of transduction factors involving Th2 cytokines expression has revealed a prominent decrease of the mast cell specific transduction factors including NFAT-1, NFAT-2, c-Jun, and NF-${kappa}B$ p65 but c-Fos. Conclusions In conclusion, the anti-allergenic activities of FF may be strongly related to the regulation of transcription factors NFAT-1, NFAT-2, c-Jun, and NF-${kappa}B$ p65 causing inhibition of Th2 cytokines in mast cells.