The effects of reaction temperature, partial pressure of methane, catalyst weight and gas hourly space velocity (GHSV) on methane decomposition were reported. The decomposition reaction was performed in a vertical fixed-bed reactor over 8Co-2Mo/Al 2 O 3 catalyst. The experimental results show that these four process parameters studied had vital effects on carbon yield. As revealed by the electron microscopy and Raman spectroscopy analyses, the reaction temperature and GHSV governed the average diameter, the diameter distribution and the degree of graphitization of the synthesized carbon nanotubes (CNTs). Also, an evidence is presented to show that higher temperatures and higher GHSV favored the formation of better-graphitized CNTs with larger diameters. The effects of reaction temperature, partial pressure of methane, catalyst weight and gas hourly space velocity (GHSV) on methane decomposition were reported. The decomposition reaction was performed in a vertical fixed-bed reactor over 8Co-2Mo / Al 2 O 3 catalyst. The experimental results show that these four process parameters studied had vital effects on carbon yield. The revealed by the electron microscopy and Raman spectroscopy analyzes, the reaction temperature and GHSV governed the average diameter, the diameter distribution and the degree of graphitization of the synthesized carbon Also, an evidence is presented to show that higher temperatures and higher GHSV favored the formation of better-graphitized CNTs with larger diameters.