Ion implantation technique is useful method to dope atoms in a thin layer to make nanoparticles. However, the thermal annealing is required for recovering ion-induced damages and growing NPs. Therefore, the redistribution of implanted atoms in the layer is very important in the formation process of nanoparticles in size and position. We have investigated the redistribution of Ag atoms implanted in a thermally grown 25-nm-thick SiO2 layer on silicon substrate. Ag negative ions were implanted at 10 keV in the thin oxide layer, where the projected ranges are calculated 12 nm. Samples were annealed at a temperature of 800 degrees or less for 1 h in vacuum with Ar gas flow (50 ml/m). Depth profiles of implanted atoms were investigated by high-resolution Rutherford backscattering spectrometry (HRBS). The formed nanoparticles in the layer were studied by high-resolution cross-sectional transmission electron microscope (XTEM). The samples implanted with Ag negative ions at 10 keV with 5 x 10 boolean AND 15 ions/cm boolean AND 2 were measured by HRBS at scattering angle of 80 degree with He ions at 400 keV. Ag atoms distributed at the surface and at a depth corresponded to the calculated profile after annealing at a temperature below 500 degrees C. It is expected that the surface accumulation of Ag atoms resulted from thermal diffusion of implanted atoms during implantation. At 500 degree 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 degree C, the main peak of concentration was appeared at 20 nm in depth, in which FWHM was 7 nm. The XTEM observation showed that the Ag NPs aligned at the same depth of 20 nm along the interface of SiO_2/Si, and that they were nanocrystals (NCs). This mono-layered Ag NCs well corresponded to the HRBS spectra. Thus, we have formed almost mono-layered nanocrystals for Ag implantation in thin silicon dioxide layer.