The objective of this study was to determine the effects of digesta viscos- ity on growth performance, apparent ileal digestibility (AID) of nutrients, metabolizable energy (ME) and N-corrected ME (MEn) of diets, organ parameters and intestinal modulation in broiler chickens. The experiment was designed as a randomized complete block design with body weight (BW) used as a blocking factor. On d 8, a total of 168 broiler chickens (186.8 ± 20.5 g) were allocated to 3 corn-soybean meal-based diets with increasing concentrations of carboxymethyl cellulose (CMC) – 0, 10, or 20 g/kg added at the expense of cornstarch from the basal diet. There were 8 replicate cages per dietary treatment with 7 birds per pen. Birds were euthanized on d 22 for sample collection. Data were analyzed using the GLM procedure of SAS. Orthogonal polynomial contrasts were used to estimate the linear and quadratic effects of CMC. The ileal and jejunal digesta viscosity linearly increased (P < 0.01) with dietary CMC addition. There was a linear reduction (P < 0.01) in the BW of birds on d 15 and 22 with CMC. Moreover, the BW gain and the gain-to-feed ratio of birds linearly reduced (P < 0.01) with CMC addition throughout the experimental period. With CMC inclusion, the duodenal, jejunal, and ileal length linearly increased (P < 0.05). The addition of CMC also linearly increased (P < 0.05) the relative weight of the ileum and proventriculus in birds. The AID of dry matter, nitrogen, ME, and MEn linearly decreased (P < 0.01) with CMC inclusion. In the jejunum, the digesta crude mucin quadratically reduced (P < 0.05). However, there was a tendency for a quadratic increase (P < 0.1) in the jejunal villus height and villus height to crypt depth ratio with CMC addition.