This study investigates the transport resistance of fouling layer formed during concentrating the extract of traditional Chinese medicine (TCM) by direct contact membrane distillation (DCMD). Empirical equations for calculating the physical properties of TCM extract were combined with the heat transfer equations of DCMD to describe the concentrating process These equations were solved through an iterative computer program to obtain the thermal resistance of fouling layer and interface temperatures Based on the experimental and calculated results, factors affecting the thermal resistance of fouling layer were identified and discussed. It was found that membrane fouling in this study was mainly caused by the deposition of suspended solid particles in TCM extract A porous fouling layer covers the membrane surface, introducing an additional thermal resistance to overall heat transfer in DCMD For a batch operated concentrating process, the flux declines faster with increasing feed temperature and feed flow velocity, due to the faster increase of foulants concentration in the feed However, the comparison of the relationships between fouling resistance and concentration ratio obtained under different conditions reveals that the growth of fouling layer can be effectively limited by increasing either the feed temperature or feed flow velocity (C) 2010 Elsevier B V All rights reserved