Abstract: Multi-wall carbon nanotubes (MWCNTs) with controlled diameters were synthesized by an improved-floating catalyst chemical vapor deposition method,using toluene and ferrocene as a carbon source and catalyst precursor,respectively.Ferrocene was sublimed in a heater and carried as a gas mixed with toluene vapor into a reactor where MWCNTs were formed.The effects of the sublimation temperature,hydrogen content in the gas mixture and gas flow rate on the diameter and diameter distribution of the CNTs formed were investigated.Results indicated that the CNT diameter distributions could be controlled by changing the sublimation temperature.The higher the sublimation temperature,the narrower the distribution of CNT diameters.The average CNT diameter decreased and levelled off with increasing hydrogen content in the gas mixture from 0 to 40 vol%.The CNT diameter decreased with increasing gas flow rate.