The purpose of this study was to reveal the effect of the sublayer structure on the separation performance of the thin film composite (TFC)-type forward osmosis (FO) membranes. A novel TFC-type FO membrane with a supporting layer of polyvinylidene fluoride (PVDF) porous membrane, a sublayer of carbon nanotube (CNT) network and an active skin layer of polyamide (PA) was prepared by a simple filtration-assisted interfacial polymerization method. The results showed that the prepared FO membrane exhibited the improved separation efficiency, with twice the water flux of the FO membrane without this CNT sublayer. The characterization of the prepared FO membrane structure revealed that the presence of the CNT sublayer was favorable for providing a three-dimensional free space, which increased the effective area of the PA layer by optimizing the space structure underneath the PA layer. At the same time, the experimental results demonstrated the negative effects of the conventional supporting layer structure on the separation performance and concentration polarization of the FO membrane, and highlighted the important role of the sublayer structure between the active skin layer and the supporting layer for the performance improvement of FO membranes. (C) 2017 Elsevier B.V. All rights reserved.