Abstract: Biguanidinylated chitosans having antibacterial activities were prepared through acylation between aroyl chloride with biguanidinyl group and chitosan. para-Biguanidinyl benzoyl chloride was prepared through the reaction between para-biguanidinyl benzonic acid hydrochloride and SOCl2. O-(4-Biguanidinyl benzoyl) chitosan hydrochloride (p-BGBC) was obtained through the condensation reaction of para-biguanidinyl benzoyl chloride with chitosan using MeSO3H-DMSO as reaction medium. The stru-cture, molecular weight and degree of substitution (DS) of p-BGBC were characterized by FT-IR and 1H-NMR or determined by GPC and element analysis, respectively. The minimum inhibition concentrations (MIC) of p-BGBC against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were determined by agar plate method. The results indicated that the MIC value of p-BGBCs against S. aureus was varied from 32 μg/mL to 8 μg/mL with the DS increased from 16.2% to 44.7% while that against E. coli was varied from 64 μg/mL to 16 μg/mL. When the DS of p-BGBC achieved or exceeded 36.8%, its antibacterial activities against S. aureusand and E. colithe were higher than that of Bromo-Geramium. The introduction of biguanidinyl into the chitosan could improve its antibacterial activities obviously, and the increase of DS would result in the improvement of antibacterial activities of p-BGBC