Objectives : It has been recently shown that Piperis Longi Fructus (PLF) is involved in the reduction of eosinophil recruitment and production of Th2 cytokines in vivo. However, the main therapeutic mechanisms of PLF remains a matter of considerable debate. To investigate the therapeutic mechanisms of PLF, we examined the influence of PLF on regulatory T cells number, IgE, histamine production in vivo and Th1/Th2 cytokine balance in vitro. Methods : All mice were immunized on two different days (21 days and 7 days before inhalational exposure) by i.p. injections of 0.2 $mell$ alum-precipitated Ag containing 100 ${mu}g$ of OVA bound to 4 mg of aluminum hydroxide in PBS. Seven days after the second sensitization, mice were exposed to aerosolized ovalbumin for 30 min/day on 3 days/week for 12 weeks(at a flow rate of 250 L/min, 2.5％ ovalbumin in normal saline) and PLF (150 mg/kg) were orally administered 3 times a week for 8 weeks. Splenocytes from C57BL/6 mice at 8 weeks of age were stimulated with anti-CD3 (1 mg/ml) plus anti-CD28 (1 mg/ml) antibody for 48hrs. IL-4 and IFN-$gamma$ in the culture supernatants were measured by ELISA Results : The suppressive effects of PLF on asthma model were demonstrated by the increase the number of regulatory T cells and by reducing IgE, histamine production in vivo and modulation of Th1/Th2 cytokine balance. Conclusions : These results indicate that PLF has a deep inhibitory effects on asthma model mice by increase the number of regulatory T cells, and by reducing IgE, histamine production.