In this study, the influence of reverse draw solute permeation on the rejection and prediction of NO3- and NH4+ by forward osmosis (FO) were investigated. The rejection of NO3- and NH4+ were experimentally compared between FO mode (active layer facing the feed water) and RO mode (hydraulic pressure applied). Under similar water fluxes, the rejection ratio of NO3- by HTI-ES membrane was much higher during RO than it was during FO, while the opposite was true for HTI-TFC membrane. For both membranes, the rejection of NH4+ was higher during RO than that it was in FO. Rejection was mathematically predicted by using the solution-diffusion model with the permeability obtained from the diffusion cell. The predicted NH4+ rejection for the HTI-TFC membrane matched well with the experimental rejection ratios. However, the model greatly overestimated the rejection ratios of NO3- by HTI-ES membrane and underestimated the rejection of NH4+ by HTI-ES membrane and NO3- by HTI-TFC membrane. The electrostatic equilibrium at the interface and electrostatic gradient across the membrane may differ between RO, FO and diffusion cell mode, which should be considered in future FO models.