The aim of present study was evaluate effect of various levels of dietary onion powder on immunological and blood serum biochemical parameters of Common carp (Cyprinus carpio). Fish were offered formulated diets that either contained three levels of onion powder 0.5%, 1% and 2% or the basal diet without the onion for 8 week. Blood samples were collected from caudal vein of apparently healthy fish at the end of trial. Biochemical parameters (total protein, albumin, cholesterol, triglyceride and glucose), serum enzymes activity (ALP, ALT and LDH) level and immune responses (serum lysozyme activity, alternative complement activity (ACH50) and Ig level) were assessed. The results indicated that feeding dietary 0.5% onion powder showed a significant increase in serum total protein (P<0.05) compared with those fed the diets supplemented with onion while there were no significant difference in other biochemical parameters. Subsequently, serum enzymes (ALP and ALT) activity level were significantly lower in 0.5% onion powder fed fish (P<0.05). In addition, lysozyme activity showed a significant increase in 0.5% onion group compared to the other onion and control group (P<0.05), although no significant difference in serum Ig level and alternative complement activity (ACH50) level between treatments were found (P>0.05). The results of this study demonstrated that dietary onion powder at the level of 0.5% could be an improvement in biochemical parameters and immune function of Common carp.