Oxidative stabilization of polyacrylonitrile (PAN) precursor fibers is a vital step in producing carbon fibers. The precursors mostly are prepared from different copolymers containing acidic and ester active groups to amend the fiber structure as well as cyclization and oxidation mechanisms. But the other way to improve the homo-PAN precursor quality is using the mixed or composite neat fibers. In this work, we have studied the synthetic polymers (such as polyacrylic acid (PAA) and polyvinyl alcohol (PVA)) and carbon nanotubes influence on oxidative stabilization process of homo-PAN fibers. Introducing the carbon nanotubes to PAN precursors is well-known approach to improve the mechanical properties of carbon fibers resulting from PAN thermal treatment procedure. The original method of introducing MWNT via preliminary dispersing in dilute homo-PAN/DMSO solution was developed leading to homogeneous distribution of individual nanotubes in the dope. It was shown that this approach significantly changes the structure of composite fibers, that results in decrease of stabilization temperature and integral calorific effect and leads to less local overheating during thermal stabilization process, therefore to higher product yield and more perfect product structure. The next step of this work consisted in addition to the homo-PAN dopes of low concentration of other polymers to understand a possibility of replacement of the copolymer on mixture of two or more polymers containing active functional groups. Combined systems of PAN and PAA in DMSO are mutual solutions with almost the same viscosity as individual PAN solutions and could be spun easily by traditional methods. In the case of PVA, the mixed dope is emulsion with PVA/DMSO drops as disperse phase. Nevertheless, mixed fibers precursors have been prepared successfully. It has been found that additives of other polymers do not change the precursor structure significantly, but result in changes of cyclization mechanism.