The influence of synthesis time on the synthesis of single-walled carbon nanotubes (SWCNTs) over Mo-Fe-MgO catalyst by decomposing CH4 in Ar was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Very short and isolated SWCNTs with poor quality and high defect level were obtained and the carbon yield rate was low at 2 min. The SWCNTs were longer and the graphitization degree was higher as the synthesis time was increased. At 30 min, long bundles of SWCNTs with lower defect level were obtained, and most of the growth had taken place within 30 min. After this time, nearly no SWCNTs were generated without the presence of the active catalytic sites, but amorphous graphite was still deposited due to the continuous decomposition of CH4. Carbon yield rate increased strongly with time up to 10 min. However, with longer growth time, the increase of carbon yield rate slowed, and the change of carbon yield rate was small after 30 min. The growth mechanism of SWCNTs over Mo-Fe-MgO catalyst was proposed according to the experimental results. The solid state nano-particles with high melting point on the surface of molten Fe-C droplets may be the active catalytic sites and play an important role in the formation of the SWCNT nucleus. The precipitation of carbon from the saturated Fe-C droplets leads to the formation of tubular carbon solids with an sp(2) structure. (c) 2006 Elsevier B.V. All rights reserved.