Medium composition was optimized for the production of xylanase by Aspergillus niger KK2 using statistical experimental designs. Corn steep liquor (CSL) and industrial yeast extract (IYE) were the most important factors affecting xylanase activity. The medium that produced the optimum conditions for the production of xylanase contained 3% rice straw, 1% wheat bran, 6.3% CSL, 0.15% IYE, and 0.5% $KH_2PO_4$. After 4 days of cultivation under optimized conditions in a 2.5-L stirred tank reactor, the activity and productivity of xylanase were 620 IU/mL and 6,458IU/L h, respectively. The highest xylanase activity obtained using the optimized medium was 80% greater than the activity obtained using basal medium. The xylanase activity predicted by a polynomial model was 670 IU/mL.