In this research the effects of oat Avena sativa extract on the non-specific immune system of common carp (Cyprinus carpio) was examined. For this purpose, the fishes (average weight 9.91 +/- 1.52 g) were fed with 5 g kg(-1), 10g kg(-1) and 20 g kg(-1) oat extract supplemented diets for 60 days. Following 60 days of feeding, the fishes were injected with Aeromonas hydrophila and mortalities were recorded. Lysozyme and myeloperoxidase activity, improved in all groups that received feed supplemented with oat extract. Serum glucose and cholesterol decreased while total protein and albumin in fish increased with the use of the diet supplement with oat extract. Haemoglobin (Hb), mean cell hemoglobin (MCH) and mean cell haemoglobin concentration (MCHC) increased with diet supplemented with oat extract. Oat extract at the concentration of 10 g kg(-1) showed significantly higher relative percentage survival (67%) when compared with the control against A. hydrophila injection. Also the dietary supplementation with oat extract caused a significant increase in growth parameters (final weight (FW), weight gain (WG), specific growth rate (SGR) and feed conversion ratio (FCR), when compared to non-supplemented control basal diet. The results suggest that A. sativa extract can be used as a feed supplement to enhance fish immune response and disease resistance against A. hydrophila.