In this study, the immunostimulatory effect of dietary Aloe vera crude extract was investigated in Cyprinus carpio. Three hundred fish were randomly divided into four groups. The first group was immunized with Aeromonas hydrophila bacterin (A.h) and was fed a diet contained 0.5% Aloe vera. The second group was immunized with A.h and fed a diet without Aloe vera. The third group was not immunized and fed with a diet that contained 0.5% Aloe vera. The fourth group remained as the control group and was neither immunized nor fed with Aloe vera supplements. Blood samples were taken every 14 d for eight weeks and samples were analyzed for hematological and immunological parameters. White blood count (WBC), red blood count (RBC), packed cell volume (PCV), lysozyme activity, serum bactericidal activity, complement activity, total protein, IgM concentration and specific A. hydrophila antibody were assessed. At the end of treatment, 20 fish from each group were challenged with A. hydrophila. WBC value, antibody level, lysozyme and bactericidal activity were significantly increased in the serum of fish treated with Aloe vera (p<0.05). No significant differences were seen in the RBC, PCV or complement activity among the groups. The relative percent survival (RPS) was found to be increased in fish fed with Aloe vera. This study indicates that the oral administration of Aloe vera is able to enhance some specific and non-specific immune responses in the common carp.