Polyamide reverses osmosis membranes are widely used for desalination of brackish and sea water. Membrane fouling is the most important problem facing in reverse osmosis process. Surface grafting of the polyamide membranes has been considered by many researchers to improve the anti-fouling properties of the membranes. According to the published results, it seems that surface modification of the membranes can be considered as a potential method of partial or in some cases total elimination of fouling. To this aim, chitosan was grafted for the first time on the surface of thin film composite (TFC) polyamide reverse osmosis membranes. Grafting was completed by the reaction between the carboxylic acid groups existing on the surface of the membrane and the amine groups of chitosan. 1-Ethy-3-(3-dimethyl amidopropyl) carbodii-mide (EDC) was used to induce the carboxylic groups. Effects of modification time on the properties of the membrane were investigated by carrying out the grafting process for different times of 1, 3, 5, 6, 12, 24, 36, and 48 hour. Maximum grafting density of 0.0525 was achieved for 24 hours grafting.The membranes were characterized by ATR-FTIR, AFM, SEM, and contact angel analysis methods. The results obtained from ATR-FTIR analysis showed reduction in the intensity of the peak seen at the wave number of 1730 cm -1 , attributed to C=O stretch of ester group, which confirmed grafting of chitosan on the membrane surface. AFM analysis revealed that the average surface roughness decreased from 173.9 for the unmodified membrane to 114.3 for the samples modified for 24 hour. Decreasing the contact angle to 32? for the membrane modified for 24 hour, in comparison with 52? for the virgin membrane, proved significant hydrophilicity enhancement by surface modification. SEM analysis showed no significant change in membrane surface morphology after modification. Membrane performance in water flux and NaCl rejection was improved by increasing grafting time. The maximum flux of 128 was obtained for the samples modified for 12 hours, whereas surface modification for 24 hour caused the maximum salt rejection of 99.30%. The fouling resistance properties of the modified membranes against NaCl and two different charged surfactants, namely sodium dodecyl sulfate (SDS) and N-Cetyl-N,N,N-trimethyl ammonium bromide , were compared with that of unmodified membranes. In the case of NaCl, it was found that flux decline due to the membrane fouling and the flux recovery for the samples modified for 12 hours were similar to the original samples. For the fouling tests carried out with N-Cetyl-N,N,N-trimethyl ammonium bromide , it was clearly seen that the samples modified for 24 hours showed better antifouling properties than those of the unmodified membrane. On the other hand, the unmodified membrane showed relatively better performance in comparison with the modified samples when they were tested by SDS solutions. Finally, the antibacterial activity was evaluated with E.coli .It was found membranes grafted with Chitosan for 24 hour would reduce the bacterial ratio to less than 97.5% compared to unmodified membrane. .