To study the antitumor effect of phlomiol extracted from Phlomis younghusbandii in vivo and in vitro. The inhibitory effect of phlomiol on two kinds of human tumor cells proliferation was assayed by MTT method. Transplant tumor models of S180 and H22 were used. After transplantation, different doses of phlomiol were given to the mice for 14 days. The inhibitory rates were calculated. MTT method was used to assess the proliferation of T spleen lymphocyte cells and the activity of NK cells in tumor-bearing mice with S180. Phlomiol (50-100 mg x L(-1)) inhibited the proliferation of three kinds of tumor cells in vitro, antitumor effect of phlomiol was in a dose-dependent manner (r = 0.989, P < 0.05). The inhibitory rates of phlomiol (2.5, 5, 10 mg x kg(-1)) were 28.5%-65.0% and 35.0%-74.5% in tumor-bearing mice with S180 and H22 respectively, It could stimulate the spleen T-cells in tumor-bearing mice with S180 and increase the activity of the NK cells. Phlomiol could inhibit the proliferation of three kinds of tumor cells in vitro, present antitumor effect on the tumor-bearing mice, and improve the immunological function.