Filled or un-filled multiwalled carbon nanotubes (CNTs) used in this study have been synthesized by the floating catalyst method and fixed catalyst method, respectively. The thermal stability of filled/un-filled carbon nanotubes has been investigated by using Thermogravimetric analysis (TGA) and Derivative thermogravimetric (DTG) analysis. In this report, we have developed a methodology to distinguish between filled and un-filled carbon nanotubes. Filled-CNTs are found to be more resistant to oxidation than the un-filled carbon nanotubes. The calculated activation energy of as-grown filled CNTs, by using differential method, determined to be 3.29 ± 0.04 eV, which is higher than that of highly ordered pyrolytic graphite (HOPG). Carboneous impurities; amorphous carbon, catalyst and CNT of different diameter, which are structurally different, are identified by their reactivity and the resistance to oxidation.