A one-pot approach was established to effectively convert polypropylene (PP) and linear low density polyethylene (LLDPE) into carbon nanomaterials (CNMs) including cup-stacked carbon nanotubes and carbon nanofibers under the combined catalysis of HZSM-5/NiO or HZSM-5/Ni2O3 at 700 degrees C. The effects of HZSM-5 content, a nickel catalyst, and the carbon precursor on the morphology, microstructure, phase structure, and thermal stability of CNMs were investigated with scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, Raman spectroscopy, and thermal gravimetric analysis. The effect of HZSM-5 content on the degradation products of PP and LLDPE was analyzed by gas chromatography and gas chromatography mass spectrometry. With increasing HZSM-5 content, the yields of light hydrocarbons and aromatics increased due to proton acid sites on the surface of HZSM-5 catalyzing cracking of PP or LLDPE degradation products. This one-pot approach provides a novel potential way to largely transform mixed waste polyolefin into high value-added CNMs.