The effect of feeding barley-based diets (%60) supplemented with four commercial enzymes containing xylanase and ?-glucanase activities were investigated on performance and gastrointestinal size of broiler chicks. The enzyme supplemented diets were compared with unsupplemented barley- or corn-based diets. A total of 234-day-old male broiler chicks (Ross 308) were allocated to 6 treatment groups, with 3 replicates per each treatment group, 13 birds per replicate pen, and the data analyzed in a randomized complete block design. All the enzymes in supplementation with the barley-based diet significantly (P<0.05) increased Average Daily Feed Intake (ADFI) of birds in the grower period, while in the entire period, only enzyme C was significantly (P<0.05) effective on ADFI. Addition of all enzymes to the barley-based diet significantly (P<0.05) improved Body Weight Gain (BWG) and Feed Conversion Ratio (FCR) of broilers in comparison with feeding unsupplemented barley diet in starter, grower as well as in the total period. In contrast, no significant effect was observed in finisher period by enzyme supplementation (P>0.05). As well, in case of the mentioned parameters, no significant differences were observed among any of the enzymes in any period. At 42 days of age, relative weight of pancreas significantly (P<0.05) decreased only by enzymes A, C, and D, but the relative weight of liver and gizzard was not significantly (P>0.05) affected by enzyme supplementation. However, almost all the enzymes employed in the experiments significantly (P<0.05) decreased the relative weight and length of different parts of intestine. The results of the current study show that, addition of all four different enzymes are effective in overcome anti-nutritiving the effect of NSP on broiler performance