In this study, chitosan was successfully grafted onto the top surface of a bromomethylated poly(phenylene oxide) (BPPO) ultrafiltration membrane without pretreating the membrane at harsh conditions and/or using other cross linkers. Due to the grafting of polar groups of chitosari onto the membrane top surface, the hydrophilicity of the membrane top surface and the polar component of the total surface energy are improved compared to the pristine BPPO membrane. Theoretical calculation shows that the polar component of surface energy is a major contributor to the reduction in interfacial free energy of the membrane surface and interaction strength between foulants and membrane surface Therefore, the foulants adsorbed onto the top surface of chitosan/BPPO composite membranes are much easier to desorb during the cleanning process and as a result, a higher flux recovery was obtained compared to the pristine BPPO membrane, Moreover, due to the antibacterial nature of chitosan, such composite membranes show a better antibacterial property and the antibacterial rate was improved by 70% in comparion with pristine BPPO membrane.