In this study, the mechanism was studied of membrane fouling that occurred when charged nanoparticles (NPs) are present in the feed solution of the forward osmosis (FO) process. Three NPs were used that were positively, neutrally, and negatively charged, respectively, at pH 6.0: zinc oxide (ZnO), titanium dioxide (TiO2) and silicon dioxide (SiO2). The FO membrane used was a negatively charged polyamide-based flat sheet. The fouling behavior arising in the presence of each NP type varied according to electrostatic attraction and repulsion between the NPs and the membrane; this affected the water flux and reverse salt flux (RSF). Positive NPs accelerated membrane fouling close to the membrane surface by their electrostatic attraction with the membrane, leading to the formation of a thick fouling layer that acted as a path for salt ion transfer, thereby increasing RSF. On the other hand, negatively charged NPs were electrostatically repelled by the membrane surface and thus, prevented the NPs from attaching easily to the membrane surface, thereby reducing fouling and leading to increased water flux and decreased RSF. To rectify this mechanism, the positively charged NPs was altered to negative by changing the pH; this enhanced water flux and reduced RSF.