Abstract: To study effect of mannose oliogosaccharides (MOS) on juvenile crucian carps initial body weight of (16.190.03) g, a total of 360 juvenile crucian carps were randomly divided into five groups, the control group was fed with basal diet, the others were the treated groups fed with basal diet supplemented with 0, 60, 120, 240 and 480 mg/kg dose of MOS, respectively. After rearing 80d, juvenile crucian carp was infected with Aeromonas hydrophila and the growth, hepatic immune parameters, serum immune parameters, intestinal structure of juvenile silver crucian carp were investigated. The results showed that there were no significantly effects on growth index at 50 days and 80 days after feeding. The serum globulin concentration for 50 days after food intake in the MOS group was higher than that of the control. After 80 days feeding, 240 and 480 mg/kg doses of MOS improved the serum alkaline phosphatase activity, 120, 240 and 480 mg/kg doses of MOS group enhanced the serum globulin content, 480 mg/kg doses of MOS group enhanced the total anti-oxidative capacity, and the MOS treatment group improved the villus length of intestion compared to the control. There was no significant difference in the level of HSP70 mRNA for 80 days feeding in liver. The infection with A. hydrophila caused mortality in all groups, but the control was the highest. The present study suggested that feed supplementation with 240-480 mg/kg doses of antibacterial peptides had the potential to stimulate immunity, increase resistance of pathogenic infection in crucian carps.