Nanosized silica hybrid silver complex (NSS) showing strong antifungal activity, in which nanosilver (nano-Ag) was bound to silica (SiO2) molecules, was synthesized via gamma-irradiation at room temperature. NSS was characterized via field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDXS), ultraviolet-visible (UV-Vis) spectrophotometer, and thermogravimetric analysis (TGA). The FESEM images and EDXS data showed that well-dispersed 3-to-10-nm Ag nanoparticles (core part) were loaded onto the outer parts of 5-to-20 nm SiO2 nanoparticles. The antifungal efficiency of NSS was evaluated against Rhizoctonia solani, Botrytis cinerea, and Colletotrichum gloeosporioides. In the case of Rhizoctonia solani, the growth rate was decreased typically by more than 90% at a 6 microg/ml concentration of NSS as a medium additive. The antifungal-action mechanism was investigated via transmission electron microscopy (TEM) analysis of the NSS treatment against Botrytis cinerea. The stability and antimicrobial activity of NSS were determined, using the plate culture method, from several water samples containing NSS after 7-day NSS treatment. Moreover, the NSS solution maintained stable antifungal activity for at least 24 mos. These results suggest that NSS, an environment-friendly nanomaterial, can be used as strongly effective growth inhibitor of various microorganisms, making it applicable to diverse antimicrobial-control systems.