This study was conducted to investigate the effect of adding insoluble fiber at different rearing periods and its combination with acidifier on performance and gastrointestinal changes of broiler chickens. For this purpose, 1250 male broiler chicks were used in a completely randomized design with 10 treatments and 5 replicates of 25 each. The experimental treatments were: A) basal control group (pellet feed from 0 to 42 days), B) basal diet with acidifier from 0 to 42 days, C) basal diet with 3% rice hulls during 0 to10 days and fed with basal diet from 11 to 42 days, D) basal diet with 3% rice hulls + acidifier during 0 to 10 days and fed with basal diet + acidifier from 11 to 42 days, E) basal diet with 3% rice hulls during 0 to 24 days and fed with basal diet from 25 to 42 days, F) basal diet with 3% rice hulls + acidifier during 0 to 24 days and fed with basal diet + acidifier from 25 to 42 days, G) basal diet with 3% rice hulls during 0 to 42 days, H) basal diet with 3% rice hulls + acidifier from 0 to 42 days, I) basal diet with 0.8% arbocel from 0-42 days, J) basal diet with 0.8% arbocel + acidifier from 0 to 42 days. One kg/ton acidifier was added to the diets. The average daily feed intake and daily gain were not significantly affected by dietary treatments during starter, grower, finisher and total period. Only at 24 days there was significant difference in the feed conversion ratio between F and (C and control) treatment (P 0.05). There was no significant difference in the villus width, crypt depth, villus height to crypt depth ratio, muscle width and villus surface area between treatments at 24 day of age. There was a significant difference in the length of jejunum (as a relative live body weight) between D, I and J than the control treatment (P 0.05). The relative weight of the gizzard in birds fed with 3% rice hulls + acidifier during 0-24 days of age (F) was significantly increased as compared with control (P 0.05). Furthermore the highest and lowest pH of gizzard and ileum were recorded in the control group and those fed with rice hulls during 0-42 days as well as arbocel + acidifier G and J treatments having lowest pH in gizzard and ileum respectively (P 0.05).In addition, at 42 day of age, treatment with 3% rice hulls + acidifier from 0-42 days (H) showed the higher villus height to crypt depth ratio (P 0.001). The lowest crypt depth were recorded in treatments which fed by only rice hulls during 0-10 (C) and rice hulls+ acidifier during 0-42 days (H) (P 0.00005). Dietary treatments didn’t show significant difference on other gastrointestinal tract parameters. Although broiler chicks treated with acidifier was a higher Lactobacillus population than other treatments, there was no significant difference between the treatments in the E. coli population. In general, the adding of 30 kg/ton of rice hulls to broiler chicks diet resulted in the highest possible weight and the lowest mortality rate. Keywords: Broiler Chickens, Rice Hull, Arbocel, Acidifier, Gastrointestinal Tract