Pressure retarded osmosis (PRO) operated in counter-current flow mode is more efficient than in co-current flow mode to extract salinity gradient energy. Knowledge of the performance of counter-current flow PRO under various operating conditions (on equilibrium or off equilibrium) is of paramount importance to understand the potential capacity of the technology and to optimize process design. In this study, a systemic and rigorous numerical procedure was developed for performance simulation of counter-current flow PRO. An optimization technique was used to accurately determine the originally unknown flow rate of the draw solution at the feed entrance of membrane channel so that the procedure could also be used for PRO systems not at equilibrium. With this numerical procedure, new interesting findings were made about the ideal counter-current flow PRO. A characteristic parameter of the PRO, the required membrane area to reach equilibrium for any given operating condition, was determined and reported for the first time. Another exciting finding was that the no-flux zone (dead region) occurs adjacent the draw entrance at the critical feed fraction when the membrane area is greater than the required equilibrium area. Power density and specific energy in PRO under various conditions were investigated with this numerical procedure.