The removal of Cs(I) from radioactive wastewater by three forward osmosis (FO) membranes was investigated using NaCl solution and simulating seawater, as draw solution. The influences of membrane material, membrane orientation, draw solution concentration and flow velocity on FO performance were examined. Removal efficiency of Cs(I) by FO was compared with other membrane separation processes as well. Active layer played an important role in Cs(I) retention. CTA membrane achieved high Cs(I) retention of 90.35%-97.15%. TFC membrane was unable to reject Cs(I) effectively with Cs(I) retention of lower than 48%. Support layer significantly affected water flux. ES support layer helped to alleviate concentration polarization and therefore yield high water flux. Effect of membrane orientation was related to the support layer. AL-FS obtained higher Cs(I) retention than AL-DS by CTA-NW. AL-FS featured lower Cs(I) retention than AL-DS by CTA-ES and TFC-ES. NaCl concentration influenced Cs(I) retention by TFC-ES at AL-DS mode. Increasing flow velocity could not enhance Cs(I) retention at both modes. The optimal operational conditions for Cs(I) retention were achieved by CTA-ES membrane with NaCl concentration of 1.5 M and flow velocity of 11 cm s(-1) at AL-DS. Under the optimal condition, Cs(I) retention reached 96.24%. Therefore, CTA-ES membrane is a potential membrane for Cs(I) removal from radioactive wastewater by FO process.