Wrapping of a single-walled carbon nanotube (SWNT) was examined by using a poly[(m-phenylenevinylene)-alt-(p-phenylenevinylene)] (PmPV) derivative. The polymer's intrinsic ability in forming a helical conformation was found to play an essential role in the separation of nanotubes. Among about 15 tubes present in the pure SWNT (HiPcoTM) sample, the polymer was found to selectively pick up the tubes (11,6), (11,7) and (12,6), which correspond to tube diameters of 1.19, 1.25 and 1.24 nm, respectively. The SWNTs of smaller diameters were held loosely by the PmPV, and were gradually dropped out under centrifugation. The suspension solution prepared from the SWNT and PmPV was not permanently stable, with precipitation occurring after a few weeks. Irradiation in the UV-vis region exhibited a catalytic effect to shorten the precipitation time to hours. Those tubes, which were held loosely by PmPV, were quickly separated from the suspension during the irradiation process.