In this research the effect of feeding wheat/ barley based diets supplemented with three commercial enzymes containing xylanase and ?-glucanase activities and including matrix value their on performance and gastrointestinal tract size of broiler chicks was investigated. A total of 260-day-old male broiler chicks (Ross 308) were allocated to 5 treatment groups, with 4 replicates per treatment and 13 birds per replicate in a RCB design. Results showed that only addition of enzymes B and C to wheat/ barley based diet increased average daily feed intake significantly (P<0.05). Addition of all enzymes to wheat/barley based diets significantly (P<0.05) improved body weight gain and feed conversion ratio (FCR) of birds in comparison with feeding unsupplemented wheat/barley based diets. At day 43, the pancreas relative weight was significantly decreased by addition of all enzymes to wheat/barley based diet (P<0.05), but, that of liver and relative length and weight of duodenum, jejunum, ileum and cecum was not significantly affected by enzyme supplementation (P>0.05). However, relative weight of abdominal fat was significantly (P<0.05) increased only with addition of A and B enzymes. The result of current study showed that, addition of all three enzymes improve FCR similarly in diets and regarding final body weight, enzyme C had the best performance. Form the economical point of view the based on total rearing period the treatments of enzymes A and B were better than other treatments.