Abstract: Abstract: Multiwall carbon nanotubes were prepared by the catalytic decomposition of acetylene at 700℃ with Fe-Ni as catalysts and resultant carbon nanotubes were purified by removing most of amorphous carbon through air oxidation at high temperature. The purifying temperatures were chosen according to the purity of the multiwall carbon nanotubes. When the samples were oxidized at 440℃ for more than 3h in air, most of amorphous carbon was removed and the purity was greatly improved. TEM observation showed that the process cannot remove metals and their oxides in multiwall carbon nanotubes.