¹Department of Animal Science, Faculty of Agriculture and Natural Resources, University of Mohaghegh Ardabili, Ardabil, IRAN DOI : 10.9775/kvfd.2018.19719 The study was performed to investigate the effect of hydroalcoholic extract of different thyme species on performance, intestinal morphometry, nutrient digestibility and immune response of male broiler chickens. A total of 160 day-old Ross 308 broiler chicks were randomly allotted to five treatments including a control basal diet or diets supplemented with 0.1% of four thyme extracts (Thymus daenensis, T. kotschyanus, T. lancifolius, and T. transcaspicus) with four replicates of eight birds each. The experiment was lasted for 28 days. There were no significant effects of treatments on broilers performance during the first three weeks of age. At fourth weeks of age and throughout the experimental period, the chickens on T. daenensis extract showed the best weight gain among the treatments (P<0.05), while feed intake and feed conversion ratio were not affected by the treatments throughout the experimental period. The birds receiving T. daenensis extract had higher jejunal villus height and villus height to crypt depth (P<0.05) than those on the control. All thyme extracts increased villus surface area (P<0.05). Supplementation of diet with T. daenensis extract improved digestibility of organic matter, dry matter, crude carbohydrate, crude ash and ether extract (P<0.05). The immune response to Newcastle disease (ND) vaccine and sheep red blood cell (SRBC) were not affected by the treatments. The results indicated that the response of broiler chickens to thyme extract depends on birds" age and genotype of the plant. Keywords : Chicken, Digestibility, Mucosal morphology, SRBC, Thyme spp.