The　toxicity　of　polyethylene-glycol　functionalized　(PEGylated)　multi-walled　carbon　nanotubes　(MWCNTs)　and　non-PEGylated　MWCNTs　in　vivo　was　evaluated　and　compared.　Mice　were　exposed　to　MWCNTs　by　intravenous　injection.　The　activity　level　of　glutathione,　superoxide　dismutase　and　gene　expression　in　liver,　as　well　as　some　biochemical　parameters　and　the　tumor　necrosis　factor　alpha　level　in　blood　were　measured　over　2　months.　The　pathological　and　electron　micrographic　observations　of　liver　evidently　indicate　that　the　damage　caused　by　non-PEGylated　MWCNTs　is　slightly　more　severe　than　that　of　PEGylated　MWCNTs,　which　means　that　PEGylation　can　partly,　but　not　substantially,　improve　the　in　vivo　biocompatibility　of　MWCNTs.