The fetal toxicities of multi-wall carbon nanotubes (MWCNTs) with various sizes were compared in CD1(ICR) mice. MWCNTs were suspended in 2% sodium carboxymethyl cellulose solution in phosphate-buffered saline. On day 9 of gestation, dams were administered a single intraperitoneal dose of MWCNTs (4 mg/kg body weight), while dams in the control group were administered vehicle (10 mL/kg body weight). The rectal temperatures of the dams were monitored 2 h after administaration to asses statuses of the dams. The dams and fetuses were examined on day 18 of gestation. The number of live fetus per dam decreased in some MWCNTs-administered groups. The mean percentages of live fetuses in total implantations in the MWCNTs-administered groups markedly varied from 0% to 95%, and the highest mean percentage of live fetuses in the MWCNTs-administered group was equivalent to that of the control group. The decrease in live fetuses depended on an increased number of early dead fetuses. In the groups with markedly lowered rectal temperature after administration, the fetal loss were evident. The blood levels of interleukin-6 and/or monocyte chemoattractant protein-1 in dam 2 h after administration of MWCNTs markedlyr increased, especially in the goups with significant decrease in live fetuses. These results indicated a relationship between inflammation in the dam, which probabely depended on the particular length of the MWCNTs, and the fetal toxicioty of MWCNTs in mice.