Nanomaterials have been used to create unique devices at the nanoscale level. However, the toxicities of nanomaterials have not been fully tested and the risk of nanomaterials has been raised as an emerging issue in these days. In this study, the cytotoxicity of silver nanoparticles was tested using cultured mouse leydig cells. As results, silver nanoparticles showed cytotoxicity with the generation of reactive oxygen species (ROS). With the increased level of ROS, intracellular glutathione level was decreased. DNA fragmentation and caspase-3 activation suggested the apoptotic mechanism of cell death in leydig cells treated with silver nanoparticles.