Antimicrobial silver nanoparticles (NPs) were successfully synthesized on the surface of silk fibers via gamma-ray irradiation. The products were characterized with scanning electron microscope (SEM), energy dispersion spectrum, and X-ray diffraction. The results revealed that the silver particles with a diameter of less than 20 nm were immobilized and well dispersed on the Surface of silk fibers. The antimicrobial capability against the grain positive bacterium Staphylococcus aureus and the washing stability of the silk fibers produced with different conditions were tested and found to be excellent. The silk fibers treated with 1 mM solution and 10 kGy gamma-radiation showed 96%, antimicrobial activity and still kept above 85% antibacterial activity after 10 washing cycles. Moreover, a mechanism for the formation of silver NPs on silk fibers under gamma-radiation was generally discussed. The resulting silk fibers coated with silver NPs can be useful as functional fabrics in a range of applications. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2511-2515, 2009