In the aspect of wastewater treatment and reuse, the FO process has recently gained attention. However, it is a critical limitation that the FO process exhibits low removal efficiency for ammonium, which is a small hydrated radius and positively charged contaminants. To prevent ammonium leakage, an amphoteric modification of the FO membrane using [2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (MEDSAH) was used in this study. The prevention of ammonium leakage by amphoteric FO was investigated depending on the feed solution chemistries, such as ammonium concentration, feed pH, and temperature. Because of the under 60 % of ammonium rejection, the ammonium leakage is high in FO but modifications using amphoteric property were effective in reducing the ammonium leakage about 77 % than virgin FO by quaternary amines of amphoteric ion. Increasing the ammonium concentration, feed pH, and temperature enhanced ammonium leakage. Among the feed chemistries, the increased ammonium concentration is most critical for ammonium leakage in amphoteric FO. During the three-day operation of the virgin and amphoteric FO processes using real wastewater, the amphoteric FO showed less ammonium leakage by 21 % than the virgin FO. The amphoteric FO membrane reduced the deposition of organic matter by 50 % compared to the virgin FO membrane. And also, the reduction of water flux by the foulants in amphoteric FO was less by 10 % than in virgin FO. This study identifies the mechanism of ammonium transport in the amphoteric FO using feed chemistries and the evaluation of the amphoteric FO performance in real wastewater treatment was derived.