The dissociation and isomerization reactions of methyl-nitramine(MNA) confined inside armchair CNT(5,5) single-walled carbon nanotube were investigated by using the ONIOM (B3LYP/6-311++G:UFF) method. The results showed that some geometries of the confined MNA were modified by the CNT(5,5) in comparison with the structure of the isolated MNA. By analyzing the relevant structures and energies involved in the dissociation and isomerization reactions, we found that the transition state structures of the isomerization reactions to form CH(3)NHONO (R1) and CH(3)NNOOH (R2) were modified by the confinement of CNT(5,5). However, this confinement does not evidently affect the transition state structure of the HONO elimination reaction (R3). In addition, no transition state was found for the N-N bond dissociation (R4) of the isolated MNA, but this dissociation process occurred via a transition state for the confined MNA. When MNA was confined inside CNT(5,5), the activation energies of R1, R2, and R4 were decreased obviously but the energy barrier of R3 was increased slightly. The order of activation energy for these four initial reactions was also changed by the confinement of CNT(5,5). Furthermore, it was found that the relative energies of the intermediates formed by the isomerization and dissociation of MNA were also modified by the confinement of CNT(5,5). These intermediates become more stable in the confined case than in the isolated case. It was concluded that the initial reactions of MNA could be modified evdiently by confinement within a carbon nanotube.