The complex potential energy surface for the unimolecular isomerization and dissociation of nitromethane (CH3NO2), including 10 CH3NO2 isomers, 46 interconversion transition states, and 16 major dissociation products, is probed theoretically at the G2MP2/B3LYP/6-311++G(2d,2p) level of theory. The geometries and relative energies for various stationary points are determined and are in good agreement with the available experimental values. Based on the calculated G2MP2 potential energy surface, the possible nitromethane unimolecular decomposition mechanism is discussed. It is shown that the most feasible decomposition channels for CH3NO2 are those that lead to (CH3)-C-2 + (NO2)-N-2, (CH3O)-C-2 + (NO)-N-2, H2C=O + HNO, and HCNO + H2O, respectively. Among them, (CH3)-C-2 and (NO2)-N-2 are produced by the direct C-N bond rupture of nitromethane, while the formation of the latter three products is initiated by CH3NO2 rearranging first to methyl nitrite or to aci-nitromethane. The C-N bond dissociation energy for nitromethane is calculated to be 61.9 kcal/mol, lower than the nitromethane --> methyl nitrite and nitromethane --> aci-nitromethane isomerization barriers by 2.7 and 2.1 kcal/mol, respectively. Our results suggest that the CH3NO2 isomerization pathways are kinetically disfavored in view of the relatively high activation barriers, in excess of 60 kcal/mol. The nitromethane decomposition occurs either via the C-N bond rupture or via concerted molecular eliminations.