A series of experiments have been done to investigate the role of methane partial pressure in the synthesis of carbon nanotubes by catalyst chemical vapor deposition. It is supposed that there is a critical methane partial pressure (similar to 0.4 atm) for single-walled carbon nanotube (SWNT) synthesis at 850 degreesC. When the methane partial pressure is higher than the critical value, the whole rate is decided by the carbon diffusion rate on the catalyst surface or in the bulk, contrariwise, the synthesis rate is proportional to the methane partial pressure. (C) 2003 Kluwer Academic Publishers.