Novel multi-branched carbon nanotubes (BCNTs) have been synthesized using alkali-element modified Cu/MgO catalyst. It was found that alkali-element doping to the catalyst played a key role in the synthesis of CNTs, signifying a novel approach to modify Cu catalyst. Based on the transmission electron microscopy observations, a simple model is developed to describe the growth of BCNTs. (c) 2005 Elsevier B.V. All rights reserved.