The carbon nanostructures, including single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and carbon nanofibers (CNFs) were synthesized by a floating catalyst method. Carbon nanotubes (CNTs) and CNFs in different diameters and structures can be obtained by controlling the ferrocene (catalyst precursor)/ benzene (carbon source) mole ratio. The diameter of CNFs and CNTs increases while ferrocene/benzene mole ratio decreases. The general tendency is: (a) SWNTs can be synthesized when ferrocene/benzene mole ratio is around or above 15.1%. (b) MWNTs can be produced when ferrocene/benzene mole ratio is between 4.2'% and 8.76%. (c) CNFs can be synthesized when ferrocene/benzene is below 4.21%. (C) 2003 Elsevier Science B.V. All rights reserved.