In this study, we investigated the toxic effects of nanocarbon blacks (NCBs) with different sizes to mouse macrophage RAW264.7 cells. MTT and fluorescence-based LIVE assays demonstrated that NCBs uptake caused a size and dose-dependent growth inhibition to the cells. Optical microscopy observations and (99m)Tc radionuclide labeling techniques were used to investigate the cellular uptake of NCBs with different sizes qualitatively and quantitatively, respectively. Results showed that the cellular uptake amounts of NCBs increased with their increasing size. Large quantities of internal NCBs induced oxidative stress and nuclear damage in cells; these effects may be the critical factors involved in the cytotoxicity of NCBs. The implications associated with these findings are discussed.