The present work introduced a novel conceptual design of integrating Forward Osmosis (FO) membrane with the Multi Stage Flashing (MSF) or Multi Effect Distillation (MED) thermal desalination processes. A simple mathematical model was developed here to estimate the performance of the FO membrane system. A previously developed program, VDS, for estimating the performance of thermal processes was updated to include the FO system. The verified VDS program [1] was applied to simulate the performance of the FO-MSF/MED hybrid system at different recovery rates varied from 16% to 32%. Brine reject from the thermal desalination processes was recycled and used as a draw solution to reduce the cost of FO membrane pretreatment. Seawater was used as the donor solution in the FO membrane. The simulation results showed that the FO pretreatment, successfully, reduced the concentration of multivalent ions in the feed solution to the MSF and MED. It was found that the concentrations of Ca2+, Mg2+, and SO42- ions, which are responsible for scale problem in MSF, decreased with increasing the recovery rate of FO membrane. In case of FO-MED hybrid system, the thickness of the CaCO3 scale layer was calculated at different FO recovery rates. The estimated thickness of CaCO3 scale layer was 74 mu m, 43 mu m, and 39 mu m for 0%, 20%, and 32% FO recovery rate respectively. It was also found that the thickness of CaCO3 scale layer decreased in the direction from effect 1 to effect 6 due to temperature drop. Finally, the study demonstrated the feasible application of FO membrane in the pretreatment of seawater to reduce the concentration of multivalent ions which are responsible for the scale problem in the thermal desalination processes. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.