The mechanisms for ultrasounds (USs) at the frequencies of 28, 45 and 100 kHz on ultrafltration (UF) and membrane cleaning were investigated in dead-end UF cell with dextran aqueous solution. Permeate flux and resistances at different US frequencies were recorded and calculated in terms of resistance-in-series model, respectively. The flux enhancements by US at the frequencies of 28 and 45 kHz were significant, whereas no noticeable effect at 100 kHz was found. US irradiation primarily affected the concentration polarization and cake layer resistance, leading to a reduction of the total resistance and an enhancement of the permeate flux. Recovery of flux after the membrane cleaning by US with low frequency of 28 or 45 kHz was higher than that by 100 kHz. Experimental results also indicated that the flux enhancement by US at 28 kHz was stronger than that at 45 kHz at low pressure, whereas increasing the pressure, particularly to 1.2 bar, the enhancement induced by US at these two frequencies US was quite similar. In conclusion, low frequency US, such as 28 kHz can significantly enhance the flux of UF and the membrane cleaning process which could effectively decrease the concentration polarization and the cake layer resistance. ; Department of Applied Biology and Chemical Technology