This experiment was conducted to investigate the effects of different sources of insoluble fiber on perform-ance, gastrointestinal tract characteristics, and nutrient digestibility in the broiler's ileum, and cecum. A total of 380-day-old chickens of ross 308 unsexed were arranged into 5 treatments, 4 replicates, and 19 chickens in each, by completely randomized design (CRD). Experimental treatments included 1: control (corn-soybean meal) 2: arbocel (synthetic fiber) 1% in diet, 3: sunflower hulls (3% in diet), 4: soybean hull (3% in diet), 5: processed wheat straw (3%in diet). Observed parameter include: performance (feed intake, feed conversion ratio and body weight gain), gastrointestinal pH, gastrointestinal viscosity and digestibility of dry matter and protein in ileum and cecum and microbial population in ileum and cecum. The results have shown that the inclusion of 3% sunflower hulls in diets leads to the higher average feed intake from 1 to 10 days of age rather than control (P<0.05). In comparison between the ileum and cecum showed that the higher viscosity by Arbocel® in the cecum and the lowest viscosity showed by wheat straw in the ileum (P<0.05). On the other hand, the digestibility of protein and dry matter in the ileum was higher than the cecum (P<0.05). According to the results of this study, it could be noted that all fibers are considered in-soluble fibers and their different behavior in the gastrointestinal tract. The diet Arbocel® has generally led to physicochemical changes (digesta pH and viscosity) and microbial populations. It could be the most im-portant reason for these observations which is related to insoluble fiber particle size and their processing. In addition, reduced dry matter and protein digestibility in the cecum in comparison to the ileum status. Fi-nally, digest pH, viscosity, dry matter and protein digestibility were reduced by insoluble fiber in cecum in comparison to ileum. پرونده مقاله