Ultrasound effects on permeate flux and rejection of solute were studied by using two types of polyacrylonitrile (PAN) ultrafiltration (UF) membranes reinforced with non-woven cloth. The membrane was set in cross-flow UF cell and the filtration was carried out by permeating 1 wt% dextran solutions under 30 kPa until steady-state flux was obtained. Then, ultrasound with 45 kHz of frequency was irradiated oil the UF cell in water. It was found that the ultrasound irradiation significantly increased the permeate flux of different molecular weight (MW:) dextran solutions. Evidence was presented that ultrasound irradiation was effective to enhance the membrane permeate flux for dextran solution.