Making use of chitosan (CS) and ethylenediaminetetraacetic acid (EDTA) as a reaction system, CS-EDTA nanoparticles were synthesized through a facile counterion complex coacervation method. $Ag^+$ could enter porous CS nanoparticles synthesized with this method, allowing Ag nanoparticles within chitosan nanoparticles were synthesized by reducing silver nitrate with chitosan. Because of the noncovalent interaction between CS and EDTA, the EDTA could be easily removed via dialysis against water, and pure core/shell-type Ag/CS nanoparticles could be obtained. The nanoparticles showed higher antibacterial activity toward E. coli than the active precursor Ag nanoparticles and CS.