The effects of dietary supplementation of grape seed extract (GSE) and vitamin C on carcass characteristics, gut morphology and ileal microflora in broiler chickens exposed to chronic heat stress were examined. Experimental diets included a control diet (without additive), 3 levels of GSE (150, 300, 450 mg/kg of diet), and vitamin C (300 mg/kg of diet). Each of the five diets was fed to 5 replicates of 12 male Cobb-500 chicks, from 1 to 42 d of age. The birds were exposed to chronic daily heat stress of 34 ± 1 ˚C with 65-70% relative humidity for 5 hours per day from 29 to 42 d of age. Addition of GSE or vitamin C to the basal diet did not affect the percentage by weight of the edible carcass, breast, drumstick + thighs, liver, empty gizzard, or abdominal fat. Furthermore, in heat-exposed broiler chickens the relative length of duodenum, jejunum and ileum did not show significant alteration when compared to the control group before and after chronic heat stress. Dietary GSE and vitamin C caused differences in jejunum muscle layer thickness, villus height, villus width and crypt depth of birds before heat stress. Addition of the GSE or vitamin C did not affect the jejunum muscle layer thickness, villus width, crypt depth and villus: crypt ratio of broilers under heat stress (42 d). However, broilers fed a diet containing GSE had higher villus compared to the control group at 42 d (P<0.05). Adding GSE or vitamin C reduced ileal coliforms and the Escherichiacoli population of broilers before heat stress condition (28 d). Under chronic heat stress conditions, broilers fed diet supplemented with GSE had lower ileal coliforms and Escherichiacoli populationscompared to control birds (P<0.05). Thus, GSE can be recommended as a new herbal extract supplement to the diet of broiler chickens in order to improve jejunum villus height and decrease detrimental bacteria in the ileum of birds before and during chronic heat stress condition. Manuscript profile