We demonstrated an efficient improvement of activity of Ni catalysts by combining with Mo and Mg elements, by which massive growth of multiwalled carbon nanotubes (MWNTs) with straight and double helical structures have been achieved from polypropylene (PP) by means of a one-step combustion method. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) are applied to characterize the morphology and the purity of products.