Forward osmosis (FO) membranes have gained increasingly attention due to their potential advantages for various FO-based wastewater treatment, seawater desalination and power generation. In the present study, an attempt was made to systematically investigate the effects of substrate structure on the formation of the selective layer in a two-step preparation - a phase inversion for a hollow fiber substrate followed by interfacial polymerization for the ultra-thin selective layer, and on the FO performance in non-ideal situations, i.e., the feed stream contains foulants. Experimental results revealed that the substrate surface structure is very important for fabricating a RO-like thin film. A substrate with <300 kDa MWCO is preferred to obtain a good semipermeable skin based on current study. It may not be appropriate to use DI water and a low concentration draw solution (i.e., an ideal situation) to evaluate the FO membrane, as the water permeation through the RO-like skin is the rate-determining step, and the effect of the substrate structure cannot be fully reflected in this ideal situation. In addition, the surface structure of the FO membrane facing the feed is critical as it is directly associated with the tendency of FO membrane fouling, and a relatively dense membrane substrate surface is desirable in the membrane orientation of active layer facing draw solution. An ideal FO membrane should also possess anti-fouling capability or less fouling propensity in addition to high water permeability, low salt permeability and minimized structural parameter. (C) 2011 Elsevier B.V. All rights reserved.