This experiment was conducted to examine the effect of different levels of oak acorn (Quercus brantii) as a substitution for in-feed antibiotics (IFA) on growth performance, carcass traits, morphological responses of the small intestine and humoral immune responses in broiler chickens. Three hundred, 1-day-old broiler chicks (Ross 308) were allotted into 5 groups in a 6 wk study. The dietary treatments consisted of a corn-soybean coccidiostat-free basal diet as a control, control + 2 mg lincomycin/kg, or control + 10, 15 or 20 g oak acorn/kg. Dietary supplementation of lincomycin or 10 g oak acorn/kg significantly (P<0.05) enhanced final body weight of broilers at 42 d. Overall daily feed intake during the trial was higher (P<0.05) in broilers fed diets containing lincomycin or different levels of oak acorn compared with broilers fed basal diet. During the whole experiment feed:gain was significantly (P<0.05) better in broilers fed basal diet or basal diet supplemented with 10 g oak acorn/kg compared with other treatments. Carcass yield was significantly (P<0.05) higher in broilers fed diets containing 10 g oak/kg compared with other groups. In the duodenum, broilers fed diets containing antibiotic or 10 g oak acorn/kg had significantly (P<0.05) higher villus height:crypt depth ratio (VH/CD) compared with other groups. In jejunum and ileum the highest (P<0.05) VH/CD was obtained in broilers fed diets containing 10 g oak acorn/kg. Supplementation with 10 g oak acorn/kg led to higher antibody titers against Newcastle, and Influenza viruses (P<0.05). In summary, the results indicate that addition of 10 g oak acorn/kg could induce favorable influences on growth performance, structure of the small intestine and immune responses of broilers and it could be consumed in broiler diets as a substitution for IFA. Manuscript profile