In this study, a model for the water flux and the salt flux for a flat sheet membrane in pressure retarded osmosis (PRO) is developed and experimentally validated. The model focuses in the effects of operating conditions (feed and draw solution concentrations, flow rate, and temperature) on PRO performance. The validation results show a good predictability of the models under different operating conditions. An improvement of the PRO performance was shown by increasing the bulk solution temperature and the flow rate. The study of the effect of these temperatures on the performance of the PRO revealed that the feed solution temperature has a stronger impact on the membrane parameter of the PRO performance comparing to the draw water temperature. However, the increase of the water flux which resulted from the increase of the feed solution temperature is accompanied by a high salt diffusion inducing a severe internal concentration polarization (ICP). The strong relationship between the water flux and the salt flux is shown to be due to the intrinsic tradeoff between permeability and selectivity.