The effect of carbon nanotubes (CNTs) on the thermal and chemical stability of polypropylene (PP) when subjected to oxidation in a fuming nitric was evaluated. The effect of CNTs on the crystalline morphology and melting and crystallization temperature of PP was studied. The results shown a thermal stability increased markedly; the decomposition temperature, increased from 293°C for pure PP to 320°C for PP with CNTs. The crystallization temperature increased perceptibly in presence of CNTs. The oxidative degradation with nitric acid produced a reduction in molecular weight; however, this negative effect was less pronounced in the PP compositions with carbon nanoparticles. The morphological changes evaluated with X-ray diffraction showed that the alpha type crystallinity remains, irrespective of the nucleating agent, and the intensity ratios between reflections peaks was taken as an indication of an increasing nucleating efficiency.