The molecular structures and thermal decomposition of nitromethane confined inside a armchair (5,5) single-wall carbon nanotube were studied by using the complete active space self-consistent field (CASSCF) and ONIOM level of theory. Results show that nitromethane confined inside a CNT (5,5) has no Cs symmetry and the C—N bond becomes short slightly. Analysis of the potential energy curves along the C—N bond length shows that a transition state exists for the thermal decomposition of nitromethane/CNT(5,5), which is different from that of nitromethane monomer (no transition state for the C—N bond dissociation). For nitromethane/CNT(5,5), the energy barrier of the transition state is calculated to be about 198 kJ·mol^-1,which is about 21 kJ·mol^-1 lower than that of the C—N bond dissociation energy of nitromethane monomer. In addition, calculated results show that the (5,5) single-wall carbon nanotube has effect on the changing in the molecular structure and electronic charge of NO₂ and CH₃ groups during the thermal decomposition process of nitromethane.