Carbon nanotubes (CNTs) may play a significant role in the biomedical field in future. Therefore, it is imperative to thoroughly investigate the toxicity and the biocompatibility of CNTs. However, the data about the toxicity of carbon nanotubes in vivo is poor or contradictory. In this study, rats were instilled with different dose mutil-wall carbon nanotubes (MWCNTs) by intravenous injection and euthanized 90d after the single treatment for histopathological study of the lungs and other organs. All rats survived within our experimental period and no large changes in their weights appeared. It is confirmed that all MWCNTs-exposed rats suffered multiple lesions. Obvious changes of the structure of lung tissue and the micrograph of MWCNTs in lung tissue were observed by TEM, while other tissues were in order. The results suggest that MWCNTs can induce lung toxicity in the expose modes of intravenous injection.