Objectives : Atopic dermatitis is a recurrent or chronic eczematous skin disease with severe pruritus. Although the pathogenic mechanisms of atopic dermatitis are yet unknown, recently hyperresponsive Th2 cells in the acute phase are reported as the important mechanisms. Cheonggisan(CGS) is used in oriental clinics for curing acute skin lesions of eczema, atopic dermatitis or urticaria. There have been no studies on the therapeutic mechanism of CGS for curing atopic dermatitis. We aimed to find out the therapeutic mechanism of CGS on atopic dermatitis, so we observed Th2 cell differentiation in EL 4 cells and $NF-kB$ p65 activation in RAW 264.7 cells. Materials and Methods : EL 4 cells were induced the increase of IL-4 mRNA expression by phorbol-12-myristate-13-acetate(PMA) and 4-tert-Octylphenol(OP) and treated with CGS extract. RAW 264.7 cells were induced the increase of cyclooxygenase(COX)-2 mRNA expression by lipopolysaccharide(LPS) and treated with CGS extract. Results : The PMA and OP induced IL-4 mRNA expression was dose-dependantly decreased in CGS treated EL 4 cells. The LPS-induced COX-2 mRNA expression was dose-dependantly decreased in CGS treated RAW 264.7 cells. Conclusion : The results may suggest that the CGS inhibits Th2 cell differentiation in EL 4 cells and inhibits $NF-kB$ p65 activation in RAW 264.7 cells.