Forward osmosis (FO) seawater pretreatment was proposed for the removal of scale ions from seawater to the thermal desalination plant. In the current study, previously developed models were applied to estimate the effectiveness of FO pretreatment in the removal of divalent ions from feed solution to MSF/MED at elevated temperatures. The simulation results showed that the water and salt flux across the FO membrane increased with increasing the seawater salinity. However, for given seawater salinity, the water and salt flux across the FO membrane decreased with increasing the FO recovery rate. it was found that the concentration of Ca2+, Mg2+ and SO42- ions increased with increasing the operating temperature in the thermal plant but decreased with increasing the recovery rate of the FO pretreatment Additionally, an FO pretreatment-MED Scale Index (FMSI) was developed to determine the required FO recovery rate and avoids scale problems at different MED operating temperatures. Initially, Ryznar Scale Index (RSI) of the feed solution was calculated for different MED operating temperatures. Then, RSI was plotted against the FO recovery rates and the desirable FO recovery rate was determined from the plot based on the operating temperature of the MED plant. The scale index was also applied to determine the required mixing ratio of NF permeate-makeup water in the NF-MED desalination hybrid system. The application of the FO pretreatment-MED Scale Index has the potential to reduce the required time and resources to determine the desirable FO/NF pretreatment ratio of feed water to the MED plant Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.