Molecular dynamics (MD) calculations have been carried out by using the CHARMM program to study initial decomposition processes in nitramines (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) polymorphs and related nitramines) in the crystalline state. Three types of simulations with different conditions were performed to investigate the effect of the crystalline stare on the decomposition processes of the molecule. When the simulation was performed with the gas phase value of the equilibrium N-N bond length but started from the crystallographic structure as an initial conformation, which mimics the course of the trajectory from the crystal state to the gas phase, large amplitude oscillations of the N-N bond lengths were observed in the trajectories. Another simulation, which takes the crystal effects into account by adjusting the equilibrium N-N bond length in CHARMM, also showed the large amplitude oscillations of the N-N bond. In this case, it was also observed that excess vibrational energy of N-NO2 is transferred to another N-NO2 moiety. These results indicate the importance of compressed N-N bonds in the crystal for bringing about the initial decomposition process of nitramines.