Both p-Mo/MgO and r-Mo/MgO catalysts and their behaviors in the process to synthesize MWNTs with or without bundles were examined. The formation mechanism of the MWNTs bundles was specially presented where the absorption of hydrogen making the molybdenum nanoparticles separated out from inside to outside of the catalyst, then forming multilayers molybdenum nanaoparticles on the surface of the support, is assigned as the main cause of bundle growth. (C) 2004 Elsevier B.V. All rights reserved.