In order to determine the min. prophylactic dose of yolk antibody for type I duck viral hepatitis, the neutralization titer of two batches of yolk antibody against type I duck viral hepatitis was diluted to 1:256, and different doses of yolk antibody were injected s.c. into Ducklings at different days of ages: 0.1, 0.3, 0.5 and 1.0 mL·duck-1 at 1-day age and 4-day age, and 0.3, 0.5, 1.0 and 1.5 mL·duck-1 at 5-day age and 7-day age, resp. The 24 h after injection, 0.2 mL (100 LD50) of duck hepatitis virus type I RP-Z strain were intramuscularized, and observed for 10 days. The results showed that the protection rate of 1-day age and 4-day age ducklings was 80%-90%, when 0.3 mL was injected. When injected 0.5 and 1.0 mL, the challenge protection rate was 100%. When 5-day age and 7-day age ducklings were injected 0.5 mL, the protection rate of challenge was 85%-90%. When injected 1.0 and 1.5 mL, the challenge protection rate was 100%. The min. preventive dose of yolk antibody against type I duck viral hepatitis was determined to be 0.3 mL·duck-1 at 1-day age and 4-day age, and 0.5 mL·duck-1 at 5-day age and 7-day age.