Xylanases encompass a broad spectrum of industrial applications like food, animal feed, textile and biofuel. This study aimed at optimization of solid-state fermentation by Aspergillus niger CCUG33991 for maximum xylanase production using low-cost agro-industrial residues 1 in tray bioreactor . SSF 2 was performed using wheat bran , sorghum stover , corn cob and soybean meal in flasks for investigation of appropriate substrate, initial pH, media composition and minerals solution . Thereafter, the obtained results combined with results of earlier experiments were used for exploring the effects of particle size, initial moisture content and temperature in tray bioreactor . OFAT method 3 was followed to select optimum level of each parameter. Next, effect of air-drying and pelleting condition were tested on enzyme stability . Finally, the extracted enzymes were subjected to zymography . The highest xylanase activity of 2919±174 U/g-IDW 4 at 48 h was achieved using wheat bran with particle size range of 0.3–0.6 mm, initial moisture level of 70%, moistened by distilled water containing 1% (v/v) glycerol and 1% (w/v) of (NH 4 ) 2 SO 4 , and controlled bed temperature in the range of 28–35 °C with surface aeration . As a result, a 2.5-fold higher and 24 h faster xylanase production in trays than in flasks was attained. The enzyme also showed a great deal of stability to air-drying and pelleting condition. Zymogram analysis confirmed multiple isoforms of xylanase. SSF in tray bioreactor is a potential method for xylanase production. Wheat bran as low-cost, time-saving and xylan-containing hemicellulosic agro-waste is preferable to use as the carbon and energy sources for maximum production of xylanase.