Silver nanoparticles were synthesized by the reduction of silver ions through indirect electrochemical-reduction method using 12-phosphotungstic with Keggin structure as reductant. The effects of the molar ratio of phosphotungstic acid to silver ions on the silver nanoparticle sizes were investigated. The morphology, composition, particle size and phase structure of the silver nanoparticles were characterized by transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), X-ray photoelectron spectra (XPS), and X-ray diffraction analysis (XRD). The antibacterial efficiency of silver nanoparticles was tested against E. coli and B. subtilis. The results show that the average size of silver nanoparticles increases with the decrease of the molar ratio of phosphotungstic acid to silver ions. The silver nanoparticles have good and steady antimicrobial property against E. coli and B. subtilis.