We examined the influence of continuous laser irradiation on as-prepared and oxidized multi-walled carbon nanotubes (MWCNTs) on the basis of Raman scattering. Differences between Raman spectra for both types of nanotube samples are shown. We evaluate the influence of the laser power density (LPD) on two main Raman modes (D and G) based on the position of the peaks, widths, and the relative intensity ID/IG ratio. Impurities and their interaction with nanotubes are crucial for interpretation of the measured effects. Oxidation, as well as the irradiation process, improves purity of samples. No structural changes of the tubes are observed. The Raman signal decrease is likely to originate from nanotube loss.