Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) are simultaneously synthesized by catalytic decomposition of CH4 over Fe-Mo/Al2O3 catalyst. High-resolution transmission electron microscopy observation shows that produced carbon materials consist of about 70% SWNTs and about 30% DWNTs. The diameters of SWNTs are in the range of 0.8-1.5 nm while the outer and inner diameters of DWNTs are in the range of 1.75-3.1 and 0.95-2.3 nm, respectively. Raman analysis indicates that the synthesized SWNTs and DWNTs have high-quality graphite structure. (C) 2003 Elsevier Science B.V. All rights reserved.