Abstract: [Aim]To evaluate the effects of duck hepatitis virus-Ⅰ(DHV-Ⅰ) on the body weight gain in duck and the effects of Silybin on it in vivo.[Methods] 86 10d old ducks,both male and female, were collected to be subjected to the test. The experiments were conducted in 7 groups: In geoup 1～3,the animals were inoculated with 1:105 diluted duck hepatitis virus infected allantoic fluids and given 0,30 and 50mg/kg.bw.d Silybin orally respectively. In group 4～6 ,the animals were inoculated with 1:5×105 diluted duck hepatitis virus infected allantoic fluids and given 0,10 and 30mg/kg.bw.d Silybin orally respectively.The group 7 was the control one treated by injecting stilled saline into the leg muscles. By the 4d after inoculation,the wein blood were drawed from the dorsal foot veins and the plasma samples were collectde and stored at -20℃ .The body weight gain were measured from 0 to 10 days after inoculation.The plasma IGF-Ⅰ,T3 and T4 consentrations were measured by RIA [Results]At the virus dose of 1:5×105 diluted virus infected allantoic fluids,the inoculations of the virus enhanced the body weight gain significantly compared with that of control (P<0.01). While 10～50mg/kg.bw.d Silybinin could counteract the effects of the virus on the body weight gain dose-dependently.The plasma IGF-Ⅰlevels showed no correlative with the body weight gain,but the T3 levels showed a same tropism with the body weight gain. [Conclution] Sublethal dose of duck hepatitis virus enhanced the body weight gain of younger ducks significantly,the Silybin could counteract this effect in vivo.