Air pressure amplitude serves as a critical control parameter of periodic pressure solid state fermentation process. Effects of different air pressure amplitudes on cellulase production by Trichoderma viride-SL were investigated. Experiments were carried out in the same fermenter as reported earlier. Under the optimum periodic pressure amplitude, 1400 IU/g CMCase activity can be obtained in the system against 450 IU/g compared with the tray fermenter.