Pressure retarded osmosis (PRO) is an osmotically-driven membrane process to utilize salinity gradient power (SGP), which is renewable energy originated from the different salt concentration between seawater and fresh water. However, PRO suffers from membrane fouling, leading to decreased water permeability and energy density. Although prediction of fouling is important for its mitigation and control, little information is available on fouling potential in PRO process. Accordingly, this study aims at the investigation of fouling propensity of PRO membranes under different conditions. Feed solutions that have different fouling potential were used in a laboratory-scale PRO system. Silt density index (SDI) and modified fouling index (MFI) were applied as indicators for assessing PRO membrane fouling. Results showed that the power density of PRO decreases with an increase of the fouling potential of the feed waters. MFI was proposed to be a fouling index for PRO because it showed a better correlation with the power density than SDI and turbidity. When MFI value is lower than 1400 s/L2, the efficiency is higher than 70%, indicating that 30% loss in energy recovery compared to the case with D.I. water. This suggests that pretreatment requirements for PRO may be determined based on MFI results.