Central composite orthogonal design was applied to quantify relations of xylanase production, loss of dry matter and change of pH with four critical variables during solid state fermentation of a mixture of wheat bran and wheat straw on which Aspergillus niger CCUG 33991 was cultivated. The studied variables included the percentage of wheat straw, wheat bran, temperature, moisture content, and fermentation time. The second-order quadratic model predicted the xylanase activity at a suitable set of conditions namely 29oC, 55% moisture content, and 50 hours of fermentation to be 1465 U/g of fermented dry matter which differed less than 6% from measured value at this set of conditions. Furthermore, the obtained models predicted dry matter loss and pH with less than 17% error for the same conditions. The results were used for xylanase production in an intermittently mixed forcedly aerated pilot. Applicability of models for prediction of pilot scale SSF performance was investigated. The results showed considerable deviations of measured trends from model predicted ones. Presence of mixing, temperature and moisture shocks and better oxygen supply are of possible reasons of deviations.