Abstract: Ag/chitosan (CS) composite colloids were synthesized by the liquid-phase chemical reduction and then Ag/CS-g-methylmethacrylate (MMA) composite was prepared by graft polymerization.The results show that Ag nanoparticles hinder the graft polymerization of CS with MMA, which results in the decrease of the graft percentage and graft efficiency in comparison with those without Ag nanoparticles.The structural characterizations verify that Ag nanoparticles disperse uniformly in the composites.The evaluations of antimicrobial activities show that the antibacterial performances of Ag/CS and Ag/CS-g-MMA composites are higher than that of Ag or CS alone.Among them, the inhibition rate of Ag/CS-g-MMA to four kinds of bacteria of E.coli, B.subtilis, S.aureus and P.aeruginosa are 96.3%、 97.6%、 93.2%、 95.8%, respectively.The antibacterial mechanisms of Ag nanoparticles and CS were studied.The antibacterial activities of Ag/CS based antimicrobial are displayed by the cooperation of Ag nanoparticles with CS.