The effects of nanoparticles toward on the male reproductive system of mice were investigated. Three sizes (14, 56 and 95 nm) of carbon black nanoparticles were intratracheally administered (0.1 mg/mouse for 10 times every week) to ICR male mice to investigate their adverse effects on the reproductive function. The serum testosterone levels were elevated significantly in the 14- and 56-nm carbon nanoparticles-exposed groups. Histological examination showed partial vacuolation of the seminiferous tubules. In addition, the effects of particle number towards the male reproductive system were investigated. The particle number controlled 14-nm nanoparticles-exposed group (14 N group, which has approximately the same particle number per unit volume as the 56-nm nanoparticles) showed fewer effects than did the 56-nm nanoparticles-exposed groups. These results suggest that carbon nanoparticle-exposure has adverse effects on the mouse male reproductive function. Furthermore, the effects of nanoparticles on the male reproductive system depend on particle mass rather than particle number.