This study applies direct contact membrane distillation (DCMD) to concentrating the extract of traditional Chinese medicine (TCM). The transmembrane flux under various operation conditions was measured in real-time during concentration process. By decoupling the factors affecting the trans-membrane flux decline, it was found that the observed flux decline throughout the process could be attributed to the membrane fouling, the reduction of water vapor pressure and the increase of transport resistance at feed side. Analysis of the combined factors was given to show in detail the mechanism of flux decline. Factors that may affect the flux level, such as feed velocity, feed temperature and pretreatment were experimentally examined. Gas bubbling or sparging was introduced into DCMD system for reducing membrane fouling, and it was found that both gas-liquid two-phase flow at the feed side and gas back-washing within membrane module are effective ways to control membrane fouling. (C) 2007 Elsevier B.V. All rights reserved.