Abstract: Background and Objective: Recently, the term “resistant starch” has been increasingly used in the literature to describe starch that escapes digestion in the small intestine together with non-starch polysaccharides. Exogenous enzymes have been employed to ameliorate these challenges. Hence, the optimum performance of Aspergillus xylanase on maize-soy bean meal has not been fully investigated. This study was designed to test the effects of Aspergillus xylanase on apparent nutrient digestibility, protein utilization efficiency, growth performance and size of visceral organs on broilers. Materials and Methods: Three-hundred-day-old mixed sex Cobb 500® chicks were randomly allocated to five dietary treatments with five replicates of 12 birds each. Dietary treatments include, xylanase (XYL) 0 (0 g kg–1), XYL10 (1 g kg–1), XYL15 (1.5 g kg–1), XYL20 (2 g kg–1) and XYL25 (2.5 g kg–1). Results: Results showed that birds fed XYL20 and 25 had higher (p<0.05) crude fiber and dry matter digestibility. Dietary treatment XYL20 promoted the highest (p<0.05) body weight gain (BWG) in the final week. Birds fed XYL20 recorded the best (p<0.05) feed conversion ratio during all phases of the feeding trial and the highest (p<0.05) BWG during the starter phase. Birds fed XYL20 had the highest (p<0.05) values for thigh, breast, wing and carcass yields. Both protein and energy efficiency ratios (PER and EER, respectively) were improved (p<0.05) for birds fed XYL20 during all phases. The small intestine lengths decreased (p<0.05) but spleen weights increased (p<0.05) as Aspergillus xylanase enzyme levels increased. Conclusion: The optimum Aspergillus xylanase inclusion levels that caused the greatest response for all measured parameters was 2 g kg–1.