Forward osmosis (FO) process as an alternative to conventional desalination techniques has attracted much attention in recent years. Since mineral salt scaling is one of the major limitations for seawater desalination, the current study focused on the behaviors of thin film composite (TFC) FO hollow fibers, which were fabricated in our laboratory, under the circumstance of calcium sulfate (CaSO4) scaling with sodium chloride as draw solution. The performance of FO hollow fiber membranes was compared with commercially available FO flat sheet membranes. Several cleaning strategies were also explored. Depending on the membrane orientation, experimental results suggested that the scaling of the FO hollow fibers was dominated by cake layer formation along with pore blocking in the orientation of active layer facing draw solution (AL-DS). The pore blocking could be a result of the penetration of crystals from the feed solution into the support matrix of the membrane, or the formation of crystals inside the porous support layer. At the same driving force, the water flux of the FO hollow fibers in the orientation of active layer facing feed solution (AL-FS) was twice higher than the water flux of the flat sheet membrane in both orientations. The water flux of scaled FO hollow fibers and flat sheet membranes in the AL-DS orientation could be successfully restored by hydraulic backwash procedure. To the best of our knowledge, this is the first study to examine the inorganic scaling of FO membranes in the AL-DS orientation. (c) 2012 Elsevier B.V. All rights reserved.