Background and Objective : Atractylodes japonica (AJ) is a commonly-used herbal medicine in Asian countries such as Korea, China and Japan. The present study was designated to evaluate the direct effects of AJ on helper T cell activities and on Th1/Th2 lineage development in vitro. Materials and Methods : Spleen cells from 8-week BALB/c mice were cultured in CR extracts containing medium without activation for 24 hours and with activation for 48 hours. CD4+ T cells were isolated and analyzed for mRNA expression levels of INF-$gamma$, IL-4, T-bet and GATA-3 by RT-PCR and secretion cytokines levels of INF-$gamma$, IL-2, IL-4, IL-5 and IL-10 by ELISA. Results : The results demonstrated that AJ had no mitogenic effects on unstimulated CD4+ T cells, but augmented CD4+T-cell proliferation upon activation with anti-CD3/anti-CD28 antibodies in a dose-dependent manner. AJ treatment significantly increased CD4+ T cell population and IFN-$gamma$ expression was significantly enhanced, while IL-4 expression significantly decreased. In addition, in vitro Th1/Th2 polarization experiments revealed that AJ enhanced IFN-$gamma$ secretion in Th1 cells, but reduced the IL-4 in Th2 cells in dose-dependent manner. Conclusion : These results suggest that AJ treatment could be a desirable alternative therapy for the prevention or correction of Th2 dominant pathological disorders, such as allergy and asthma.