To observe the effect of a novel herbal formula Jian Ya San (JYS) on duckling growth, a total of 75 ducklings (7-days old) were randomly divided into 5 treatment groups. The groups included high dose (JYS as 4% of ration), middle dose (2% of ration) and a low dose group (1% of ration). For comparison to the JYS treated groups, there was a positive control group (Bao Jian Ding as 2% of ration), and negative (untreated) control group. After treating for 5 weeks with JYS, the results indicated an increase in survival rate of all JYS groups (100%) when compared to untreated controls (86.67%). There was a significant dose-dependent increase in duckling weight gain of all dose groups (p<0.01), along with improved feed efficiency ratio (p<0.01) when compared to negative control. The thymus, spleen and bursa of Fabricius organ weights were significantly increased (p<0.01) in the high and middle dose groups and the concentrations of IL-4 were increased in the high through low dose groups (p<0.01) when compared to untreated controls. When compared to the positive control, the high and middle dose JYS groups consistently outperformed it in all categories measured. In view of these results it can be concluded that JYS has a significant effect on duckling health and would be beneficial to use as a feed additive to improve immunity and growth performance during the first 5 weeks of life.