We present a theoretical study of the decomposition mechanism of gas-phase alpha-HMX. Four distinct channels were studied using the B3LYP/cc-pVDZ level of theory. These are as follows: (i) HMX first loses an NO2 to form HMR which further breaks a C-N bond to form a chain structure and then later loses three methylenenitramines (MN, H2CNNO2) successively; (ii) the chain structure forms a 10-member ring via a rung closure step before undergoing further decomposition; (iii) HMX first eliminates an HONO, then loses two MN, and eliminates an HONO successively; (iv) HMX eliminates two HONO successively, then loses an MN, and finally eliminates an HONO. The rate constants of each elementary reaction have been calculated using the transition-state theory. The thermodynamics properties were also calculated for the stable species by employing a standard statistical thermodynamics method. Channel i was found to be the preferred decomposition pathway on the basis of the analysis of rate constants of the elementary reactions.