Carbon nanotubes (CNTs) have been a popular material in recent years, but their thermal characteristics have not been understood completely. We investigated the unique thermal stability of multi-walled carbon nanotubes (MWCNTs) and used nitric acid (HNO3) to purify MWCNTs to promote its activation energy (Ea). The study used differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), and Fourier transform infrared (FTIR) spectrometer to analyze as-grown MWCNTs and modified MWCNTs. For DSC, the heating rate was chosen to be 0.25 to 2.0 °C/min. From DSC results, Ea and exothermic onset temperature (T0) of the modified MWCNTs increased with increasing HNO3 concentration. The TGA results showed that both as-grown and modified MWCNTs’ decomposition temperatures were higher than 500°C in air. The infrared spectra of as-grown MWCNTs and modified MWCNTs have shown that the gas phase composition is CO2 after TGA linked with FTIR. By Kissinger’s corrected kinetic equation, Ea increased with increasing HNO3 concentration. Through this study, we realized that as-grown MWCNTs and modified MWCNTs are thermally hazardous materials with high potential heat of decomposition, especially under fire exposure. Thus, it is important to know the thermal hazard characteristics of material with a measure to prevent its thermal damage during perturbed situations.