Arginine and fibers may have positive effects on gastrointestinal tract and indirectly affect immune re-sponses, growth performance, intestinal morphology and microbial population. This study was conducted to evaluate the effects of dietary inclusion of arginine and rice hull on growth performance, immune re-sponses, intestinal morphology and intestinal microbial population of broiler chickens. Three hundred and sixty male broiler chickens were randomly allocated into nine dietary treatments with four replications of 10 broiler chickens based on a factorial arrangement (3×3) in a completely randomized design. Experimen-tal treatments were including basal diets supplemented with arginine (90, 100, and 110% recommended levels), and fiber (0.00, 2.50, and 5.00%). Growth performance, immune responses, intestinal morphology, tibia bone mineral content and intestinal microbial population of broiler chickens were measured. The ef-fects of dietary inclusion of rice hull, arginine, and their interactions were significant on average daily gain (P<0.05) and feed conversion ratio (FCR) (P<0.05), so that highest gain and the lowest feed conversion ratio were observed in broiler chickens fed with highest levels of arginine (110.00%) and fiber (5.00%). Dietary supplementation of arginine progressively increased immune responses (P<0.05), while inclusion of rice hull increased villus height and crypt depth (P<0.05) and decreased the concentrations of calcium (P<0.05), phosphorous (P<0.05) and magnesium (P<0.05). The inclusion of arginine (P<0.05), and fiber (P<0.05) in higher levels increased Lactobacilli population (P<0.05). In total, higher levels of fiber (5.00%) and arginine (110.00%) improved the growth performance and are suggested in diet of broiler chickens for improving growth performance. تفاصيل المقالة