The fouling mechanisms of traditional Chinese medicine (TCM) extract produced by ultrafiltration (UF) have been investigated. A laboratory scale crossflow membrane filtration unit was used to conduct the experimental study. The rejection of the four types of macromolecules present in TCM extract is more than 80% and the transmittances of the active ingredients are more than 70%. Two models were applied to quantify and analyze the fouling mechanism. A resistance-in-series model was applied to determine the main factor responsible for fouling resistance. The experimental results showed that reversible fouling resistance mechanisms, including concentration polarization and cake layer resistance, were the principal causes of the membrane fouling. A filtration-blocking model was also utilized to analyze the predominant fouling mechanism. The results showed that the flux decline during UF of the TCM extract could be well described by the combined cake-complete model. The model fit well to data measuring the permeate volume as a function of time under a wide range of operating conditions. This fit analysis indicates that the development of membrane fouling was a dynamic process that started with pore blocking at the initial stage of filtration and followed by continuous cake layer formation. (C) 2014 Elsevier BM. All rights reserved.