Ultrasonically induced cavitation has been found to be effective for looseningand detaching the cake deposited on the surface of the membrane duringmicrofiltration, thus remarkably increasing the filtration flux. In the presentresearch, cross-flow microfiltration in a plate and frame module with Nylon66membrane is investigated. The gap between the ultrasonic source and the membrane canbe varied from 1.5 to 4.7 cm, the feed velocity from 0.1 to 0.53 m/s and thetransmembrane pressure difference from 14.7 to 53.9 kPa. Two identical ultrasonicsources are attached on the outside wall of the module to transmit high energyultrasonic wave through the feed channel toward the membrane. In the experiments,baker's yeast solution (5 to 20 g/l) is filtered and the steady-state flux ismeasured. The factors affecting the filtration efficiency, i.e., the transmembranepressure, the ultrasonic intensity, the feed velocity, the feed concentration, thesonication time and the distance between the membrane and each ultrasonic source,have been studied. Experimentally, it is found under constant ultrasonic irradiation that anincrease in the transmembrane pressure results in a remarkable increase in thesteady-state flux. When the transmembrane pressure is further increased, thebeneficial effect on the flux gradually disappears because of the more compaction ofthe cake at the higher pressure which raises the cavitation threshold. Similarly, ifthe ultrasonic power is too high, the intensity of the cavitation collapse becomeslessened and the removal of the cake by ultrasonic effect is reduced. In addition, itis shown that the ultrasonic effect is more pronounced at a lower feed velocity and alower feed concentration. The appropriate distance between each ultrasonic source andthe membrane may be defined as the distance which provides the maximum effective areaexerted upon by cavitation. Finally, the experimental results have obviously shownthat the suitable application of ultrasonic wave can enhance the flux upto 3 timesthat without ultrasonic wave. The side effects of ultrasonic on the membrane andyeast have also been investigated. It is found that under the maximum intensity of3.6 W/cm('2) at the constant temperature of 30 degree C, there is no change on thesize and the growth capability of the yeast. In addition, a preliminary analysis ofthe ultrasonic application on the microfiltration system has been presented.