Carbon based nanomaterials (CBNs) such as multi walled carbon nanotubes (MWCNTs), graphene etc have attracted huge interest due to their widespread applications in contemporary nanotechnology. The ion bombardment is an important method which can be used for tuning of the structural and morphological properties of the CBNs. In the present work, we have carried out ion irradiation on MWCNTs (prepared by chemical vapour deposition method) with 5 MeV Si ions at different fluences ranging from 1 × 1014 to 5 × 1016 ions/cm2. The resulting structural and morphological modifications on the pristine MWCNTs have been characterized by using X-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy. Results observed after successful characterization are compared with pristine sample results and we observed that the irradiation creates defects in MWCNTs and causes amorphized carbon nanomaterials depending on the fluence and energy deposited by energetic ions inside the MWCNTs.