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Here multi-walled carbon nanotubes (MWNT) of different average diameters with ppm level purity were produced by a low temperature C 2 H 4 pyrolysis on Fe-Co type catalysts combined with forthcoming graphitization at 2200-2800 8C in argon flow. Annealed nanotubes were characterized with X-ray fluorescent analysis, BET surface measurements, HR TEM, X-ray diffraction, DTA analysis, and measurements of temperature and magnetic field dependences of conductivity. The graphitization of MWNT results in removal of residual catalyst metal impurities, reduction of the wall defects, and closure of nanotube tips. It was found that extent of these effects depends on MWNT diameters. It was proposed that the graphitization is caused by a significant thermal displacement and diffusion of carbon atoms at temperatures higher the Debye temperature. 1 Introduction To date, most carbon nanotubes synthesized via catalytic CVD method are applied for composite materials production. It should be mentioned that composite materials needed for high-temperature applications require pure carbon nanotubes because metal impurities can catalyze the composite destruction under these conditions. At the same time catalytic CVD method results in the encapsulation of metal particles inside the multi-walled carbon nanotubes (MWNT) core. Hightemperature treatments are traditionally used to produce high quality graphite and graphite-based materials [1][2][3]. Several research groups have investigated effects of MWNT high-temperature annealing at 1500-3000 8C [4][5][6]. It was shown that high-temperature treatment leads not only to the catalyst removing but also to the changes of tube structure and morphology [7,8].In this paper, the systematic study of influence of hightemperature treatment on MWNT structure and properties was performed. Two series of MWNT with different average diameters were used and characterized with various physical methods. It allows the technique of production of thin MWNT with ppm impurity level and with improved structure to be developed.