Several fouling indexes, such as silt density index (SDI), have been applied in the reverse osmosis (RO) process to predict the fouling tendency, design the pretreatment process, and optimize the cleaning cycles. However, the forward osmosis (FO) process has no reliable method, as yet, that relates fouling with its performance. This study utilized the concept of fouling reversibility to develop a fouling index for the FO process simulation. The osmotically driven reversibility index (ORI) was determined by measuring the flux recovery after the physical cleaning or flushing cycle. The protocols of ORI measurement were developed systematically by determining the proper operation mode, cross-flow velocity for physical cleaning, and draw solution type and concentration. The applicability of ORI was examined by investigating the FO process in treating secondary effluents from various full-scale wastewater treatment plants. The results demonstrated that the normalized final flux by fouling in FO was closely correlated to the ORI measurements with high statistical reliability (R-2 = 0.84). After conducting a long-term stable FO operation, it is deduced that the ORI is a useful evaluation parameter for continuous FO and its cleaning cycles in which periodic flushing is normally utilized to exploit and enhance the fouling reversibility of FO.