The in vivo effect of recombinant Bacillus subtilis expressed chicken interferon alpha (B. subtilis-ChIFNa) in prevention of infectious bursal disease (IBD) was investigated in 3 week old chicks. Each chicken was given a dose of 0.5x10¹⁰ spores of B. subtilis-ChIFNa with 1 or 5 supplies by eye drop or oral administration. Challenges with dose of 10⁴ ELD50 of virulent infectious bursal disease (vIBDV) per experimented chicken through oral route were carried out after 2 or 6 hours of B. subtilis-ChIFNa administration. The results showed that B.subtilis-ChIFNa could be used as a potential preventive and therapeutic antiviral agent against IBD,  B.subtilis-IFNα administration to chickens by oral route at 6 hours before vIBDV infection gave the best effect in IBD prevention