Single wall carbon nanotubes (SWNTs) without amorphous carbon coating were prepared by thermally decomposing acetylene (C2H2) at the temperature range 750-1200 degreesC in a floating iron catalyst system. The C2H2 partial pressure was controlled to make a carbon supply limiting growth of SWNTs. The higher reaction temperature above 1100 degreesC seemed not to favor the SWNT production due to the quick thermal decomposition of C2H2.