A route to producing multi-walled carbon nanotubes (MWCNTs) was reported,in which polyacrylonitrile micro-spheres (PANMSs) were assembled into one-dimensional strings of carbon spheres and transformed to carbon nanotubes at elevated temperature.It was found that the diameters of the MWCNTs are uniform and correlated with the size of the PANMSs used.Struc-tures of intermediate products obtained by ceasing the reaction have been examined by high resolution transmission electron micros-copy.The observations indicate that the formation of MWCNTs follows a different mechanism from the well-studied vapor-liquid-solid mode.It involves a direct self-assembly and solid state structural transformation of PANMSs under the promotion of nitro-gen atoms as shown by the X-ray photoelectron spectra of the resulting samples.On the basis of these observations,a sphere-string-tube mechanism was proposed for the MWCNT formation.