This study was carried out to evaluate the effect of onion extract on growth performance, carcass characteristics and some blood metabolites in Ross and Cobb broiler chickens. One hundred sixty, 1-d-old broiler chickens (mean initial weight: 36.5 ± 1 g) of male sex (Ross-308 and Cobb) were randomly conducted as randomized complete design with 4 treatment and 4 replication. The experimental groups included two treatments for Ross strain – control (RC) and the group receiving basal diet + onion extract as 1 % in drink water (RO) - and two treatments for Cobb strain –control (CC) and the group receiving basal diet + onion extract as 1 % in drink water (CO). The chickens received food and water freely for 6 weeks. The performance parameters were measured during the experimental period. Blood samplings were performed for determine of blood parameters on 42 days of age. The results showed that use of onion extract lead to significant increase (p<0.05) of yield traits in both strain. Maximum daily weight gain (49.97 g), body weight (2135.94 g), and the lowest feed conversion ratio (1.82) were related to RO group. The using of onion extract resulted in decreasing of triglyceride and glucose and increasing of HDL of blood serum in both strain. In general, the results of this experiment showed that onion extract at the level of 1% in drink water can improve the functional and some blood parameters in Ross and Cobb broiler chickens.