Antibacterial activity of chitosan was examined against Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus).Chitosan showed high antibacterial activity against E.coli and S.aureus,with higher molecular weight showed stronger antibacterial activity.The MIC was 0.05% and 0.025% (v/v) for E.coli and S.aureus respectively.In addition,this study showed that when chitosan forms nanoparticles it lost antibacterial activity.