Decomposition of dimethylnitramine (DMNA, (CH3)(2)NNO2) has been studied extensively over the past decades. Although several different mechanisms have been proposed for the initial decomposition of DMNA, the dominant decomposition channel is still far from fully understood. In this report, we collect all the results reported in the literature, along with our new experimental and theoretical results, into a single reference for a sensible comparison in order to reach a general conclusion on DMNA decomposition. In this effort, nanosecond laser, energy resolved spectroscopy and complete active space self-consistent field (CASSCF) calculations are employed. The parent DMNA molecule is electronically excited using two different UV excitation wavelengths, 226 and 193 rim, to initiate the decomposition process. The NO molecule is observed as a major decomposition product with relatively hot (120 K) rotational and cold vibrational distributions by both time-of-flight mass spectrometry and laser induced fluorescence spectroscopy. On the basis of the experimental observations, a vitro-nitrite isomerization mechanism is predicted to be the major channel of decomposition of DMNA in the excited electronic state with a minor contribution from the HONO elimination mechanism. The branching ratio between vitro-nitrite isomerization and HONO elimination channels is estimated to be approximately 1:0.04. CASSCF calculations show that surface crossing (conical intersection) between upper and lower electronic states along the vitro-nitrite isomerization reaction coordinate plays an important role in the overall decomposition of DMNA. Presence of such m (S-2/S-1)(CI) conical intersection in the vitro-nitrite isomerization reaction coordinate provides a direct nonadiabatic decomposition pathway from the Franck-Condon point of the S-2 surface, which is experimentally accessed by 226 nm photoexcitation. This excited state isomerization takes place through a loose geometry for which the NO, moiety interacts with the (CH3)(2)N moiety from a long distance (similar to 2.8 angstrom); however, in the ground electronic state, a similar (S-1/S-0)(CI) conical intersection in this vitro-nitrite isomerization reaction coordinate hinders the isomerization exit channel, rendering NO, elimination as the major thermal decomposition channel of DMNA.