The present study aimed to investigate the effect of supplementary soy isoflavone (genistein) on performance, jejunal morphology and immunological responses of broiler chicks. A total of 675 day-old male broiler chicks (Ross 308) were randomly assigned to five replicate pens (15 chicks each) of each of nine experimental diets. Dietary treatments were included a negative (not-supplemented) control diet, two positive controls (virginiamycin or zinc-bacitracin), and diets containing 10, 20, 40, 80, 160 or 320 mg/kg genistein. At d 28 of age, two randomly-selected chicks from each pen were slaughtered to evaluate lymphoid organ weights and alterations of jejunal epithelial cells. Also, two birds per pen were euthanized on the final day (42 d) to measure carcass characteristics. The results showed that antibiotic-supplemented diets and dietary levels of 20-80 mg/kg soy isoflavone (ISF) caused significant increases in feed intake during starter, grower (P 0.05), and overall (P 0.01) experimental periods. Furthermore, daily weight gain was influenced (P 0.05) by dietary treatments throughout the trial. Although the relative weights of pancreas, heart, and thigh meat were not affected, diets fortified by either antibiotic or levels of 20 or 40 mg ISF/kg resulted in improvements of breast (P 0.05) and carcass (P 0.01) yields. The birds fed on ISF-supplemented diets had bursa of Fabricius (P 0.05) and thymus (P 0.01) with more relative weights. Similarly, 10 d-assessed cutaneous hypersensivity to phytohemagglutinin-P injection and antibody titer against Newcastle disease virus were markedly higher in ISF-chicks than other experimental groups. In addition, inclusion of soy isoflavone increased (P 0.05) peripheral lymphocytes and subsequently lowered (P 0.01) heterophil to lymphocyte ratio. The lowest plasma triglycerides (P 0.01), cholesterol and LDL (P 0.05) were assigned to the groups fed on ISF-diets. Also, dietary genistein resulted in a considerable decline in liver cholesterol (P 0.05) and breast/thigh meat malondialdehyde (P 0.01). Interestingly, antibiotic-diets increased jejunal villus height and decreased crypt depth. The results on ileal digestibility showed that dietary antibiotic or ISF (particularly the levels of 20 to 80 mg/kg) supplementation caused the best digestibility coefficients for crude protein and fat. From the present findings, it can be concluded that low to medium levels of dietary genistein supplementation could beneficially affect performance and immune measurements and also increase the carcass oxidative Keywords: Broiler Chick, Lymphoid Organ, Cellular and Humoral Immunity, Carcass Oxidative Stability, Jejunal Morphology, Ileal Nutrient Digestibility