We analyzed the whole-body distribution of C-14-ADP-labeled silica nanoparticles (C-14-ADP-SiO2 nanoparticles) and submicron particles (C-14-ADP-SiO2 submicron particles) after intravenous injection into ICR mice. The C-14-ADP-SiO2 nanoparticles and submicron particles were synthesized before the injection and the particle size was 19.6 and 130 nm, respectively. Similarly, the shape was spherical and the crystallinity was amorphous. After the synthesis, we injected mice with the C-14-ADP-SiO2 nanoparticles or the C-14-ADP-SiO2 submicron particles and dissected tissues after 1, 2, 4, 8 and 24 h. The radioactivity in the tissues was measured with a liquid scintillation counter. As a result, the retention percentage in bone, skin, lymph nodes, and the digestive mixture was at least twofold higher in the C-14-ADP-SiO2 nanoparticles-exposed mice, whereas the retention percentage in the kidney was statistically higher in the C-14-ADP-SiO2 submicron particles-exposed mice. Both types of C-14-ADP-SiO2 particles mainly translocated to the muscle, bone, skin, and liver, but hardly translocated to the brain and olfactory bulb. Furthermore, the C-14-ADP-SiO2 nanoparticles had a higher retention percentage (62.4 %) in the entire body at 24-h post-injection than did the C-14-ADP-SiO2 submicron particles (50.7 %). Therefore, we suggested that the C-14-ADP-SiO2 nanoparticles might be more likely than the C-14-ADP-SiO2 submicron particles to be retained in the body, and consequently they might be gradually accumulated by chronic exposure.