Air pressure amplitude serves as a critical control parameter in periodic pressure solid state fermentation process. Effects of air pressure amplitudes on thermostable xylanase production by Thermomyces lanuginosus SD-21 were investigated. Under the optimum periodic pressure amplitude, namely: at lower limit of 0.05 MPa and upper limit of 1.5 MPa. Among the lignocellulosic substrates tested, corn cob and wheat bran supported a high xylanase (EC 18.104.22.168) secretion by Humicola lanuginosa in solid-state fermentation (SSF). Enzyme production reached a peak in 96 h followed by a decline thereafter. Enzyme production was very high, xylanase activity 8237 IU /g of dry moldy bran can be obtained in the system compared with 4520 IU/g in conventional tray fermenter, cultivation of the mold in large enamel trays yielded a xylanase titer comparable with that in flasks. Parametric optimization resulted in a 45.13% increase in enzyme production in PPSSF.