The NO2 fission reaction of gas phase alpha-HMX has been studied using a direct ab initio method within the framework of microcanonical variational transition state theory (mu VT). The potential energy calculations were calculated using the hybrid nonlocal B3LYP density functional theory with the cc-pVDZ basis set. The calculated results show that the potential energy of breaking the axial NO2 groups is lower than that of breaking the equatorial NO2 groups. No traditional transition state was found along the reaction path. Microcanonical rate constants calculation shows the variational transition state varies from 2.0 to 3.5 Angstrom of the breaking N-N bond length as a function of the excess energy. The mu VT method was used for thermal rate constants calculation over a temperature range from 250 to 2000 K. The fitted Arrhenius expression from the calculated data is k(T) = 1.66 x 10(15) exp(-18748K/T) s(-1), which is in good agreement with the experimental data at low temperatures.