The aim of the present study was to investigate the effect of garlic powder on performance of broilers (e.g. growth rate, cumulative feed intake and feed conversion rate) and immune response for Newcastle virus (NVD) disease at 7, 14 and 21 days post vaccination and avian influenza virus (AIV) disease at 22, 29 and 36 days post vaccination. One hundred and sixty Hubbard chicks at one-day-old were chosen randomly and divided into four groups (40 birds in each group). The chicks in the first group were fed on control diet free of garlic, (GP0), but the 2nd, 3rd and 4th groups received diet supplemented with 100 (GP100), 150 (GP150) and 200 (GP200) g garlic powder/tonne, respectively. Results showed that the diets containing feed additives of garlic powder had a highly significant effect on broilers' performance (P