Inclusion of cheap and fibrous feed ingredients is gaining attention in the poultry industry, which makes the understanding of the nutritional and physiological role of dietary fiber of importance. Therefore, the objective of this study was to evaluate how fiber type, particle size, and inclusion level affect the growth performance, organ development, intestinal viscosity, intestinal morphology and gene expression in broiler chickens. A total of 648 one-day old Cobb® male broilers were randomly assigned to a control diet and 8 other dietary treatments divided in 2 fiber types (cellulose vs soyhulls), 2 particle sizes (100 and 600 μm), and 2 inclusion levels (4% and 8% crude fiber). Birds were reared to 21 days of age in battery cages (n=6 replicates). On day 21, digestive organ weights were recorded for analyses of organ growth, ileal digesta samples were taken for analyses of intestinal viscosity, intestinal samples were taken for analyses of histology, and jejunal mucosas were collected for analyses of nutrient transporters. Data were analyzed as a completely randomized block design with 8 treatments organized as 2 × 2 × 2 factorial using JMP® 2021. Orthogonal contrasts were used to determine the effect of fiber inclusion compared to the control group or individually, whereas the main effect interactions were evaluated excluding the control group to be able to assess the effect of the independent variables without the variability introduced by the control group. Groups fed 8% crude fiber from cellulose (8% CL) had the lowest weight gain regardless of the particle size (P < 0.01). The control group had the highest feed intake among treatments (P < 0.01). Groups fed 8% crude fiber from soyhulls (8% SH) with a coarse particle size had the heaviest relative gizzard weight among treatments (P = 0.045). Groups fed 8% SH had the heaviest small intestine weights regardless of the particle size (P = 0.009). No differences were observed in the relative weights of the liver and ceca. The highest viscosity was observed in the group fed 8% SH with a fine particle size (P < 0.001). The group fed 4% SH with a coarse particle size had the longest duodenal villus (P < 0.001). The shortest jejunal villus height was observed in the group fed 8% CL with a fine particle size (P < 0.001). Ileal villus was highest in groups fed high cellulose levels regardless of the particle size (P < 0.001). The highest expression of peptide transporter 1 was observed in the group fed 8% CL with a coarse particle size (P = 0.008).