Oxidative stress and inflammatory responses induced by silica nanoparticles were evaluated both in mice and in RAW264.7 cell line. Single treatment of silica nanoparticles (50mg/kg, i.p.) led to the activation of peritoneal macrophages, the increased blood level of IL-1beta and TNF-alpha, and the increased level of nitric oxide released from the peritoneal macrophages. mRNA expressions of inflammation-related genes such as IL-1, IL-6, TNF-alpha, iNOS, and COX-2 were also elevated in the cultured peritoneal macrophages harvested from the treated mice. When the viability of splenocytes from the mice treated with silica nanoparticles (50mg/kg, 100mg/kg, and 250mg/kg, i.p.) was measured, the viability of splenocytes was significantly decreased in the higher dose-treated groups (100mg/kg, 200mg/kg i.p.). However, cell proliferation without cytotoxicity was shown in group treated with relatively low dose of 50mg/kg i.p. When leukocyte subtypes of mouse spleen were evaluated using flow cytometry analysis, it was found that the distributions of NK cells and T cells were increased to 184.8% and 115.1% of control, respectively, while that of B cells was decreased to 87.7%. To elucidate the pro-inflammatory mechanism of silica nanoparticles in vivo, in vitro study using RAW 264.7 cell line which is derived from mouse peritoneal macrophage was done. Treatment of silica nanoparticles to the cultured RAW264.7 cells led to the reactive oxygen species (ROS) generation with a decreased intracellular GSH. In accordance with ROS generation, silica nanoparticles increased the level of nitric oxide released from the cultured macrophage cell line. These results suggested that silica nanoparticles generate ROS and the generated ROS may trigger the pro-inflammatory responses both in vivo and in vitro.