A series of Fe–Mo/MgO catalysts with different Mo content were prepared by combustion method and used as catalysts for carbon nanotube (CNT) growth. Transmission electron microscopy studies of the nanotubes show that the number of the CNT walls and the CNT diameters increase with the increasing of Mo content in the bimetallic catalyst. The growth yield determined by thermogravimetric analysis also follows the trend: the higher the Mo content, the higher the yield of the CNTs. However, the increase of Mo content leads to the lower degree of graphitization of CNTs. A comparative study on the morphology and catalytic functions of Fe/MgO, Mo/MgO and Fe–Mo/MgO catalysts was carried out by scanning electron microscopy and X-ray diffraction. It is found that the Fe interacts with MgO to form complexes and is then dispersed into the MgO support uniformly, resulting in very small Fe nanoparticles after reduction. The Mo interacts with MgO to form stoichiometry compound MgMoO4 and relative large metal Mo particles can be generated after reduction. High yield CNTs with small diameter can be generated from Fe–Mo/MgO because the avalanche-like reduction of MgMoO4 makes the catalyst particles to be small thus enhances the utilize efficiency of Fe nanoparticles.Research highlights► Increasing the Mo content in the Fe–Mo/MgO catalysts resulted in an increase in wall number, diameter and growth yield of carbon nanotubes. ► The Fe interacts with MgO to form complex (MgO)x(FeO)1−x (0 < x < 1) and is then dispersed into the MgO support uniformly, resulting in very small Fe nanoparticles after reduction. ► The Mo interacts with MgO to form stoichiometry compound MgMoO4 and relative large metal Mo particles can be generated after reduction. ► The avalanche-like reduction of MgMoO4 makes the catalyst particles to be small thus enhances the utilize efficiency of Fe nanoparticles.