This study aimed to evaluate the effect of adding insoluble fiber to the standard and diluted diets in different phases of rearing on performance, relative weight and length of digestive organs, pH of gizzard and ileum contents, and microbial population of ileum in broiler chicks. A total of 1050 male broiler chickens were used in a completely randomized design. Birds were randomly distributed in six replicates of each seven experimental treatments. Dietary treatments consisted of: A) control (standard mash diet from 0-42 days of age), B) standard diet containing 3 percent sunflower hull in the starter period, C) standard diet containing 3 percent sunflower hull in the starter and grower periods, D) standard diet containing 3 percent sunflower hull in the starter, grower and finisher periods, E) diluted diet (97% of energy and amino acids recommendations based ross 308 nutrition specification( containing 3 percent sunflower hull in the starter period, F) diluted diet containing 3 percent sunflower hull in the starter and grower periods, G) diluted diet containing 3 percent sunflower hull in the starter, grower and finisher periods. Chickens were fed with starter (0 to10 days), grower (11 to 25 days) and finisher (26 to 42 days) diets. Daily weight gain (DWG), feed intake (DFI) and feed conversion ratio (FCR) were measured periodically. At the age of 24 and 42 days, one bird was randomly selected from each replicate, weighted and killed. After dissection, the relative weight and length of the digestive organs were measured. In addition, at the age of 42 days, the acidity of gizzard and ileum contents and microbial population of the ileum area were investigated. The results showed that during the starter period, the highest amount of DWG and the lowest FCR were assigned to a group fed with standard diet containing 3 percent sunflower hull (P 0.01). Multiple comparisons showed that adding fiber to the diets resulted in higher DFI and DWG and lower FCR (P 0.01) as compare with control group. In the grower period, the highest DFI recorded for control group or those birds fed with diluted diet containings fiber during the starter period (P 0.05). The groups fed with diluted diet containing fiber had higher DFI than consumer groups standard diet containing fiber (P 0.01). Daily feed intake in the finisher period was not affected by the experimental treatments (P?0.05). Adding fiber to the diluted diet during the starter period resulted in the higher DWG and lower FCR of broilers at the finisher time, while the inclusion of fiber in the grower and finisher period led to a lower weight gain (P 0.05). At the age of 24 days, the lowest duodenum length was observed in the group fed with diluted diet contains fiber at starter and grower period (P 0.01). The effect of dietary treatments on the length of different areas of the intestine at 42 days of age was not significant (P ?0.05). At the age of 24 and 42 days, the highest relative duodenum weight was observed by feeding the standard diet containing fiber at the starter period (P 0.01) while the lowest jejunum weight recorded for the birds that fed by the control diet (P 0.05).The pH of gizzard and ileum contents at 42 days of age, as well as the population of Lactobacillus and Escherichia coli in the ileum area did not significantly affected by dietary treatments (P ?0.05). Generally, it seems that the feeding of sunflower hull in the starter period can lead to gastrointestinal tract development, higher DWG and lower FCR. While the administration of fiber for longer times and dilution of the diet resulted in negative effect on DWG and FCR. Adding sunflower hull in the starter period increased the length and weight of the duodenum, which can have a positive effect on digestive enzymes secretions and digestive processes. Keywords : Broiler chickens, Insoluble fiber, performance, Gastrointestinal development, Microbial population