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A simple green synthesis method of Ag nanoparticles was proposed. The silver nanoparticles were formed with the protection of starch using a water-bath heating method. The Ag+ from AgNO3 solution was reduced to Ag by glucose reduction. The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The nanocomposite with a silver core and starch shell was obtained. UV-Vis absorption spectrum shows that the absorbance of the colloidal silver nanoparticles increases with the radiation time, and it keeps stable when the radiation time is more than 4 h. TEM test shows that silver nanoparticles are of spherical shape, single dispersion and about 20 nm in particle size. The silver nanoparticles exhibit good antibacterial activity against S.aureus and E.coli, measured by the inhibition ring test