Dimethylnitramine (DMNA) is a prototype system used in the investigation of the unimolecular decomposition mechanism of the nitramine-compound family. The photoinduced excited-state nonadiabatic processes and successive unimolecular dissociation of DMNA were investigated by trajectory surface-hopping dynamics at the semiempirical OM2/MRCI level. Two S-1/S-0 conical intersections (CI01 alpha and CI01 beta) were found to play essential roles in the nonadiabatic decay dynamics of DMNA. After the S-1 -> S-0 decay, the excess kinetic energy finally results in the cleavage of the N-N bond in the ground electronic state. The two reaction channels through CI01 alpha and CI01 beta show differences in molecular motions and decay features. The trajectories passing CI01 alpha can hop one or several times, and the intramolecular vibrational energy transfer in the ground state takes place before dissociation, whereas trajectories following the CI01 beta channel mainly dissociate directly after only one S-1 -> S-0 hop.