In order to investigate the immunomodulatory effects and the mechanism of a Chinese herbal medicine, Zengmiansan (Astragalus immunomodulator), on immune function of chickens, three hundred 1-day-old chickens were assigned randomly into 5 groups, i.e., the blank control group, the Chinese herbal medicine Zengmiansan (ZMS)-treated group, the cyclophosphamide group, the cyclophosphamide plus levamisole group, the cyclophosphamide plus ZMS group and the control group. All chickens were immunized with Lasota vaccine by nose-drip or eye-drop at the age of 6 days. Newcastle Disease (ND) antibody titers, growth indexes of the spleen, thymus and bursa of Fabricius, the concentrations of CD4+ lymphocytes and CD8+ lymphocytes in spleen, thymus and peripheral blood, and the apoptosis of splenocytes, thymocytes and bursa of Fabricius cells were observed at the ages of 14,21, 28 and 35 days, respectively. Our results indicated that the NDV antibody titers of chickens in the Chinese herbal medicine ZMS-treated group at the ages of 14 and 21 days were significantly higher than that of the other groups (P<0.01). The growth indexes of immune organs, the concentrations of CD4+ lymphocytes and the ratio of CD4+/CD8+ T lymphocytes of chickens in the Chinese herbal medicine ZMS- treated group at the ages of 14, 21 and 28 d were significantly higher than those of the other groups (P<0.01). The apoptotic splenocytes, thymocytes and bursa of Fabricius cells of chickens in the ZMS-treated group were significantly lower than the other groups (P < 0.01) at the ages of 14, 21 and 28 days.