BACKGROUND: Recently chitosan nanoparticles have been applied as an immunostimulant and adjuvant in biomedical fields. OBJECTIVES: In this study the adjuvant effects of nanochitosan, derived from shrimp shell's chithin, on immunogenicity of Aeromonas hydrophila vaccine in common carp were evaluated. METHODS: Totally 300 juvenile common carp (51±5.6 g, Mean ±SD) were randomly divided into four groups each contained three subgroups with 25 fishes. Groups 1, 2 and 3 were immunized intraperitoneally with A. hydrophila vaccine. No adjuvant used in group 1, but nanochitosan and freund adjuvant were used with vaccine in groups 2 and 3 respectively. Group 4 (Control) injected with normal saline instead of vaccine. Fish were immunised at days zero and 14 and fishes were reared in the same situation for 6 weeks. Serum samples were taken every other weeks and immunological parameters including WBC, serum lysozyme and bactericidal activity, serum complement activity, NBT reduction activity, total protein and globulin levels and anti A.hydrophila antibody titer were compared among the groups. At the end of experiment all groups were challenged with LD50 concentration of A. hydrophila and mortality recorded for ten days. RESULTS: WBC, serum lysozyme and complement activity, NBT activity, globulin level and anti A.hydrophila antibody titer were increased in nanochitosan and freund groups compare to control group (P<0.05). Serum bactericidal activity and serum albumin level showed no significant change among the groups (P>0.05). Challenge mortality decreased in immunised groups compare to control. CONCLUSIONS: Nonochitosan posses proper adjuvant effects which is comparable with freund adjuvant and can be a good candidate as an alternative to current adjuvant in fish vaccines.