In a new approach to the preparation of solid chitosan acetate, the dependence of solubility of chitsoan acetate on the mole ratio of acetic acid to GlcN residues of chitosan was evaluated from turbidity. The structure of the product chitosan acetate was characterized by titration and FT-IR. It was demonstrated that the chitosan acetate with high solubility retained the structure and antibacterial activity of chitosan. The antibacterial activities against Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) have been exhibited by a special kind of agar and further investigated by optical density method. It was found that chitosan acetate at a concentration of 0.1% (m/V) exhibited some antibacterial activity but some of the bacteria did still grow. With a concentration of 0.15% (m/V), there were nearly no bacteria grown. Electron micrographs revealed bacterial action patterns of chitosan acetate towards E. coli and S. aureus.