Abstract: Among the various methods of vaccination, the oral route is simple, cheap and ideal for mass administration to fish of all size. However, the bacterin of oral vaccination is digested easily by digestive enzyme in the gut so that it could not yield adequate immune responses. The major immune-responsive region of fish is in the posterior intestine, therefore, many methods are taken to protect antigen against digestive degradation. In this study, sodium alginate was used to encapsule the formalin-inactivedAeromonas hydrophila. TheAeromonas hydrophila vaccine vaccine-loaded sodium alginate microspheres were prepared using spray drying method. Two vaccines including microcapsulated vaccine(MC) and whole cell bacterin(WC) were prepared, and given to crucian carp(Carassius auratus gibelio) by oral administration at 24±1℃water temperature. After vaccination, sample blood from vein of tail and mucous from anterior intestine, mid intestine and posterior intestine were collected every week. The antibody of blood and mucous was assayed by indirect ELISA. The tissues of anterior, mid and posterior intestine were cut into pieces and studied by immunohistochemistry in a certain time. The results showed that after the fish was vaccined, its antibody titers of blood and mucus were higher and lasted longer time. The highest antibody titer of MC was 1∶80 in blood and 1∶320 in mucus of posterior intestine. Immunnoreactive positive clumps were detected in mucous layer and submucous layer using immuno-histochemistry method. Positive staining was stronger in MC group than in WC group. In the net cage test, relatively survive percentage(RSP,%) of MC group and WC group were 61. 1% and 50%, respectively. High antibodies were produced in two groups, the highest antibody titer was 1∶512 at 30 days after oral vaccine. The immune period would be 8—10 month. But there was no difference between two groups. Above study leads some conclusions:sodium alginate can be used for encapsulingAeromonas hydrophila. The microsphere vaccine ofAeromonas hydrophila vaccine can induce crucian carp both mucosal immunization and serum immunization by oral vaccination. The result of immunohistochemistry proves that sodium microspere can resist gastric destruction effectively.