Abstract: The effects of temperature, reaction time, and flow rate on the structure of carbon nanotubes (CNTs) were studied using catalytic chemical vapor deposition of methane over an Fe-Mo/Al₂O₃ catalyst. Results show that the temperature is a key parameter to control the wall thickness of the CNTs. Low temperature leads to the formation of multiwalled carbon nanotubes (MWCNTs) with wall thickness of 2–7 nm. Relatively high temperature is in favor of the growth of double-walled carbon nanotubes (DWCNTs), whereas high temperature promotes the generation of single-walled carbon nanotubes (SWCNTs). A further increase of temperature results in the generation of MWCNTs with a wall thickness of 3–15 nm and large carbon particles.