In this study, effect of ethylene and acetylene on synthesis and diameter of carbon nanotubes has been examined. The synthesis of carbon nanotubes has been carried out by chemical vapor deposition method on iron catalyst nanoparticles, at 925 °C under atmospheric pressure. Iron oxide nanoparticles have been prepared in alcohol solution containing MgO powder as a support, with a combination of 10–40 wt% by impregnation method. During synthesis of carbon nanotubes, temperature, carrier gas flow and hydrocarbon gases have been kept constant. The produced materials have been characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.