Carbon nanotubes have attracted attention not only due to electrical, optical, and mechanical applications but also due to their presence in biological and pharmaceutical products. In this study, modified multi-walled carbon nanotubes (MWCNT) were used as a model to evaluate potential subchronic effects of carbon nanotubes on mice. ICR mice were treated with phosphorylcholine-grafted multi-walled carbon nanotubes (MWCNT-PC) daily for 28 d at 10, 50, or 250 mg/kg by the intraperitoneal (ip) route. Subchronic exposure to MWCNT-PC did not produce any apparent systemic effects in mice. The body weight of the high-dose group was significantly lower than control in male mice, whereas tissue to body weight ratios of liver, spleen, and lung rose significantly with increase of dose of MWCNT-PC. There were significant differences between high-dose exposure and control groups. Accumulation of carbon nanotubes and inflammation response in liver, spleen, and lung were observed in the high-dose exposure group. No systemic toxicity and histopathological changes were found in 10-mg/kg exposure groups. Data in the present study support the view that MWCNT in vivo do not exert apparent marked effects in mice and that MWCNT products are relatively safe for human consumption.