Antimicrobial acitivity of chitosan fiber was reinforced by N-halamine which is a compound containing nitrogen-chlorine covalent bond that is formed by halogenation of amine in chitosan. Upon pH and chlorination time of sodium hypochlorite solution, aminogroup in chitosan fiber was differnetly chlorinated according to respective conditions. The chlorination of chitosan fiber was confirmed by iodometric titration. The results obtained are following: longer reaction time caused the chlorination of chitosan fibers to increase, and acidic conditions during the treatemnt was more effective compared to alkaline or neutral conditions. The chlorinated chitosan fibers showed powerful antimicrobial activity against both E. coli and S. aureus. SEM observations demonstrated that the chlorinated chitosan fibers were not damaged under the acidic treatment condition.