Objective: To observe the effect of Tripterygium wilfordii on Th1, Th2 cytokines in asthma patients for further study on the therapeutic mechanism. Methods: Twelve patients of middle or severe asthma were treated by Tripterygium polyglucoside 40 mg or 60 mg daily for 4 weeks. Blood of patients was collected before and after treatment for serum and peripheral blood mononuclear cells (PBMC) preparation. The prepared PBMCs were stimulated in vitro with Concanavalin A (ConA) for 6 hrs and followed by culturing with Triptolide for 24 hrs and then the supatant was collected. The concentration of interleukin-2(IL-2),-4(IL-4), -5(IL-5) and interferon-γ(IFN-γ) in serum and in the supatant were detected by enzyme-linked immunosorbent assay （ELISA). Results: Serum levels of IL-2, IL-4 and IL-5 of patients decreased significantly after treatment of Tripterygium polyglucoside (P＜0.01), but IFN-γ level was under the detection sensitivity both before and after treatment. Triptolide could inhibit PBMC to secrete IL-2, IL-4 and IL-5 in vitro (P＜0.01), but IFN-γ was also under the detection sensitivity. Conclusion: The marked inhibition of Th2 cytokine expression by Tripterygium was the important mechanism of it in treating asthma. But the fact that Tripterygium also showed inhibition on Th1 cytokine indicated that the inhibition of Tripterygium on Th2 and Th1 cytokines was non-specific.