Abstract: Thinwalled carbon nanotubes (CNTs) were prepared from methane decomposition using Ni/Mo/MgO as catalysts loaded with 1% mole fraction of Ni and smaller proportion of Mo. The relationship between CNT diameter and Ni/Mo ratio was studied by SEM, TEM, XRD and Raman spectroscopy. The particle size and the active phase of the catalyst could be modulated by the Ni/Mo ratio. With decreasing Ni/Mo ratio, a NiMo alloy was first formed, and then an isolated Mo phase was formed on the catalyst, as characterized by TEM and XRD. The NiMo alloy, with very small size, was responsible for the growth of thinwalled CNTs , whereas, the Mo phase was related to the formation of largediameter thickwalled CNTs. The thinwalled CNTs with outer diameter of 3.0nm and inner diameter of 1.3nm could be obtained with high selectivity, a narrow diameter distribution, and high purity by controlling the Ni/Mo ratio at 6. Raman spectra confirmed the formation of thinwalled CNTs with few defects. The formation of thinwalled CNTs was mainly attributed to a rapid surface diffusion and precipitation of carbon on the small metal alloy crystallites.