The unimolecular dissociation of azomethane is studied by first sampling the reaction channels with a density functional theory based ab initio molecular dynamics method and then by mapping out the reaction barrier and transition structure for each channel with Gaussian based ab initio method at G3 and B3LYP/6-31G(d) levels. A number of new dissociation channels are identified, and all of them involve the migration of H atoms or the breaking of a C-H bond. There is also notable difference among these channels for trans- and cis-azomethane. In the gas phase, a CH4 loss channel could be present in the dissociation of cis-azomethane. For N-N bond cleavage observed on metal surfaces, the hydrogen migration from C to N atoms, by either 1,2- or 1,3-hydrogen shifts, is the first step. Further H migration could result in N-N cleavage and produce HCN and CH3NH2, both observed in previous surface experiments. The concerted C-H bond cleavages in cis-azomethane could also produce H-2.