Formation of nanoparticles in a 50-nm-thick SiO 2 film on Si was conducted by silver negative ion implantation at 30 keV with various doses, and the state of nanoparticles in the thin oxide film was observed by a cross-sectional scanning TEM. The sample implanted with 1x10 1 5 ions/cm 2 showed nanoparticles with diameter of about 2-3 nm distributed in the middle of the SiO 2 film on Si. The location of nanoparticles was in good agreement with the calculated profile by TRIM-DYN. Nanoparticles with various diameters of 4-8 nm formed in samples of 1x10 1 6 and 1x10 1 7 ions/cm 2 . The size of silver nanoparticles was found to relate strongly to density of implanted atoms in the layer.