Experimental study of electronic structure of the initial multi-walled carbon nanotubes (MWCNTs) as well as the exposed to annealing and irradiation N-MWCNTs, was carried out by XPS and NEXAFS methods. Based on the obtained photoelectron valence band spectra and C K-edge absorption spectra, a theoretical description of the density of electronic states (DOS) near the Fermi level was proposed to explain the dependence of the DOS and the band gap on temperature, concentration of doping atoms and various atomic configurations formed in a system during synthesis, subsequent ion-beam and heat treatment of MWCNTs.