We present our ab initio results on carbon nanotubes functionalized with dichlorocarbene (CCl2). We find that the electronic properties of carbon nanotubes are significantly modified depending on the site of [2 + 1] cycloaddition. The random addition of dichlorocarbene is found to reduce the band gap of semiconducting nanotubes while those of metallic nanotubes are increased. This is related to the quasi-bound states within the energy gap originated from dichlorocarbene, which are revealed at the large supercell calculation. (c) 2005 Elsevier B.V. All rights reserved.