We have investigated thermal diffusion behavior of implanted Ag atoms in SiO2 by using a high-resolution RBS method in the formation press of monolayered Ag nanoparticles. Ag atoms were implanted by negative ion implantation at 10 keV with 5x10(15) ions/cm(2) into the 25 nm-thick SiO2/Si. Samples were annealed at 500-800 degrees C for 1 h under Ar gas flow. At annealing temperature of 500 degrees C, implanted Ag atoms distributed at the surface and at a depth corresponded to the calculated profile. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500 degrees C, the very small peak in concentration was observed at a depth of 22 nm. This means that a diffusion barrier for Ag atoms exits in this depth. The diffused atoms accumulated at this depth. At 700 degrees C, the main peak of concentration was appeared at 20 nm in depth, where FWHM was 7 nm. These results well corresponded to the mono-layered Ag nanocrystals observed by HR-TEM.