The non-isothermal TG/DTG/DSC technique has been used to study the thermal decomposition of RDX as pure and impure (contain 5 wt. % HMX) in the absence and presence of 5 wt. % irganox 1010 antioxidant under nitrogen atmosphere at different heating rates (4, 6, 8, and 10 degrees C min(-1)). The DSC curves show an exothermic peak for decomposition of RDX exactly after its melting point. The activation energy (E-a) for thermal decomposition of pure and impure RDX in the absence and presence of irganox was calculated using non-isothermal isoconversional methods of KAS, OFW, and Friedman for different conversion fraction (alpha) values in the range of 0.1-0.9. The pre-exponential factor (A) and the kinetic model have been determined by means of the compensation effect and the selected model is confirmed by the nonlinear fitting method. The activation energies for thermal decomposition of pure RDX in the absence and presence of irganox are 240.5 to 246.2 and 330.0 to 350.6 kJ mol(-1) with the reaction model of R-3 and D-2, respectively, whereas; the E-a values for decomposition of impure RDX in the absence and presence of antioxidant are 172.1 to 173.0 and 195.3 to 214.2 kJ mol(-1), respectively, with the reaction model of R-2 for both of them.