Biofouling is the undesirable bacteria build up on a membrane surface, and has been a major problem in the application of membrane technology in water and wastewater treatment. In this study, cellulose acetate was made with addition of various consentration of chitosan. The purpose of modifications was to make an antibacterial membrane which able to decrease biofouling on membran surface caused by gram-negative bacteria. The modified membranes were then examined for 2 kinds of test, first is membrane characterization test which include permeability test, FTIR, and SEM, second is antibacterial test. The antibacterial activities of chitosan against E. coli were determined by using inhibition zone method. The results indicate that chitosan is able to inhibit growth number of bacteria on the membrane surface but doesn't effective to use as an antibacterial for membrane due to weaknesess of antibacterial activity. From those various concentration, addition of 0,5% chitosan was the best compositon to inhibit the growth of bacteria.