Abstract: Aspergillus niger and Trichoderma viride are used for xylanase (EC 3.2.1.8) production using some lignocellulosic substrates under Solid State Fermentation (SSF). The results indicate that Barley Bran (BB) is the most potential substrate for higher xylanase production compared with other used substrates. The influence of various culture conditions including; fermentation period, incubating temperature, initial pH, initial moisture level, nitrogen sources, mineral sources, inoculum size and different levels of barley bran were studied. Generally, A. niger is more active in xylanase production compared with T. viride when grown on 1.0 g barley bran with moisture levels of 1:3 (75%), at pH 5.5 and 35°C after 2.0 days incubation in absence of additive nitrogen source. Parametric optimization of xylanase yielded 11.1 and 6.9 fold increase on barely bran compared with rice straw as initial solid substrate by A. niger and T. viride, respectively.