N-alkyl chitosan derivatives were prepared by introducing alkyl groups into the amine groups of chitosan via Schiff’s base intermediates. Quaternization of the N-alkyl chitosan derivatives were carried out using methyl iodide to produce water soluble cationic polyelectrolytes, novel chitosan derivatives with quaternary ammonium salt. Their antibacterial activities against S.aureus were explored by the viable cell counting method in acetate buffer(pH 6.0). The antibacterial activities of the chitosan derivatives with quaternary ammonium salt increased with increase in the chain length of the alkyl substituent, and this increased activity could be ascribed to the contribution of the increased hydrophobic properties of the derivatives.