Abstract: The use of enzymes has been common in many industries for many years. More recently, the animal feed industry began to incorporate specific enzymes in the animal and poultry feed. The main goals of enzyme supplementation of poultry diets have predominantly been related to the hydrolysis of fiber or NSP fraction in cereal grains. The purpose of the present study was to investigate the effect of exogenous enzyme supplementation of maize, wheat and barley-based diets on broiler performance. A total of 720 one day-old broiler chicks (Arian breed) were allotted at random in equal numbers in floor pens (3 pen/treatment) and fed with 8 different ingredient diets. Broiler diets were prepared based on: a standard maize based diet (M); maize plus enzyme (ME); barley plus enzyme (BE); wheat plus enzyme (WE); maize-barley plus enzyme (MBE); maize-wheat plus enzyme (MWE); wheat-barley plus enzyme (WBE) and maize-wheat-barley plus enzyme (MWBE). From day 7 onwards and repeated weekly body weight gain, feed intake, feed conversion, mortality was determined. At the end of rearing period six chickens per treatment were slaughtered and carcass yield, abdominal fat pad content, intestine`s weight and size changes were measured. The result gives an impression that no significant differences were found in terms of live weight, feed intake, feed efficiency and chick`s survivability between enzymes treated and control diets. Dietary enzymes had no effect on gizzard and liver weight, carcass yield and abdominal fat pad contents. Enzyme supplementation, however significantly influence the weight but not the size of intestine. It has been shown that adding of enzyme to the feeds based on cereal grains with higher fiber contents improve the performance of broiler chicks to the levels at least as those obtained with maize based diets. The obtained results at the present study indicate the anti-nutritive effects of SNP on the performance of broilers were overcome by adding of enzymes.