Thermal diffusion of implanted Ag atoms in 25-nm-SiO₂/Si and nanoparticle formation after annealing at various temperatures were investigated by a high-resolution RBS and cross-sectional TEM. Ag atoms were implanted into the thermal grown thin oxide layer on silicon substrate by negative-ion implantation at 10 keV with 5×10¹⁵ ions/cm². The samples were then thermally annealed at 500, 700 and 800°C for 1 h. In the HRBS measurement with 400 keV He⁺ beam, Ag atoms in the as-implanted sample were found to have almost the same depth profile as a predicted one by TRIM-DYN calculation, although some Ag atoms diffused to the surface due to local heating during implantation. After annealing at 500°C, a main fraction of Ag atoms appeared at a depth of 12 nm as same as that of the as-implanted sample, and two small precipitations of Ag atoms appeared at depths of 7 and 22 nm. After annealing at 700°, the only one fraction peak of Ag atoms appeared at a depth of 20 nm with a relatively narrow FWHM. At this temperature, the XTEM observation revealed the Ag precipitation at 700°C was contributed from the mono-layered Ag nanoparticles. The both of small and remarkable precipitations of Ag atoms observed at a depth around 22 nm just near the SiO₂/Si interface after annealing at the temperature less than 700°C, suggested the existence of a diffusion barrier against Ag atoms in the thermally grown SiO₂ on Si. This is considered to be due to compressive stress in the transition layer in SiO₂ just near the crystalline Si surface. However, after annealing at 800°C, the Ag atoms diffused to the SiO₂/Si interface at 25 nm.