A study was designed to investigate the effects of different levels of garlic powder on productive performance and immune response of broiler chickens from 0-42 days of age. 240 day-old male Ross 308 broiler chickens were randomly allocated to 6 treatments consisting of 4 replications with 10 chicks each. The groups were assigned to receive the treatment diets as follows: no added garlic powder (control group), control diet supplemented with 0.2%, 0.4%, 0.6%, 0.8% and 1.0% garlic powder. Average daily feed intake (ADFI), body weight gain (BWG), feed conversion ratio (FCR), performance index (PI), survivability and production number (PN) measured in the whole experimental period. The serum antibody level against NDV was measured by hem agglutination-inhibition test (HI) in 3 periods after vaccination in days 8 and 22 and carcass trait of broilers measured at the end of the experiment. Garlic supplementation, significantly (P< 0.05) influenced production performance of broilers at 0-14, 14-28 and 28-42 days of age, and the whole experimental period. The best ADFI, BWG, FCR and PI in overall period obtained from chickens received 0.2% garlic powder and the worst obtained from chickens received maximum level of garlic (1.0%). Carcass traits of broilers were numerically affected by high levels of dietary garlic supplementation. Serum antibody titer level against NDV in all ages were not significantly affected (P< 0.05) by dietary garlic supplementation.

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