This study investigated the onion powder supplement in the diet of the Common carp (Cyprinus carpio) on growth, corpse composition and hematological parameters. Twenty fish, averaging 18±1.9 g., were randomly stocked into tanks. Onion powder was added to the basic diet of 39% crude protein and 18% crude lipid at the concentrations of 0, 0.5, 1 and 2%, as four experimental diets. Respectively by hand at a rate of 3-5% of body weight twice daily for 45 days. Blood samples were collected from caudal vein of apparently healthy fish at the end of trial using syringe and hematological indices (WBC, RBC, Hb, PCV, Neurophyl, Monocyte and Lymphocyte) were determined. The optimal growth performance and feed efficiency was observed in fish which were fed the 0.5% onion powder diet despite of no significant difference with the control group (P>0.05). There were no significant differences in survival rate among all groups (P>0.05). Also, no significant differences was observed in crude protein of corpse (P>0.05), while total lipids and ash decreased significantly at the concentrations of 0.5% onion powder (P<0.05). No significant difference in hematological parameters between treatments were found (P>0.05), although the fish fed the 0.5% onion powder diet showed an enhancement of blood variables compared to the other groups. In conclusion, it can be suggested that dietary inclusion of 0.5% onion powder was effective at improving growth, feed efficiency, body composition and hematological parameters of common carp.