The objective of this study was to evaluate the effect of ultrasound (US) on the ultrafiltration (UF) of a natural product, Radix astragalus (RA) aqueous extract, and the cleaning process of fouled membrane. Specifically, the effects of ultrasonic frequency, power and irradiation mode on flux and resistances in UF processes were investigated. Ultrasonic irradiation had a strong impact on the normal UF process of RA extracts, especially with low frequency and high output power. An enhancement of 12-15% in permeate flux was observed when US at effective power of 10 W and frequency of 28 or 45 kHz was used. When irradiated with an US power of 120W, the fluxes increased dramatically with the increased ultrasonic intensity, nearly 70% higher in flux than those with stirring only. The intermittent irradiation mode was desirable not only for its effective flux enhancement, but also for lowering energy consumption. Upon ultrasonic irradiation, the reversible resistance, including concentration polarization and cake layer, was sufficiently reduced as revealed by the quantitative resistance analysis using resistance-in-series model. The employment of US in both mechanical and chemical cleaning processes for fouled membranes resulted in a much higher flux recovery, especially at low frequency and high power setups. The application of US is an effective and promising approach to enhance the UF process for natural products, and for both mechanical and chemical cleaning of fouled membranes. (C) 2009 Elsevier B.V. All rights reserved.