The dynamics of the initial thermal decomposition step of gas-phase alpha -HMX is investigated using the master equation method. Both the NO? fission and HONO elimination channels were considered. The structures, energies, and Hessian information along the minimum energy paths (MEP) of these two channels were calculated at the B3LYP/cc-pVDZ level of theory. Thermal rate constants at the high-pressure limit were calculated using the canonical variational transition state theory (CVT), microcanonical variational transition state theory (mu VT). The pressure-dependent multichannel rate constants and the branching ratio were calculated using the master equation method. Quantum tunneling effects in the HONO elimination are included in the dynamical calculations and found to be important at low temperatures. At the high-pressure limit, the NO2 fission channel is found to be dominant in the temperature range (500-1500 K). Both channels exhibit strong pressure dependence at high temperatures. Both reach the high-pressure Limits at low temperatures. We found that the HONO elimination channel can compete with the NO2 fission, one in the low-pressure and/or high temperature regime.