The solid phase transition in crystals prepared from molecular energetic materials under extreme conditions is important for understanding the detonation mechanisms. By applying the first principles density functional calculations, a detailed theoretical study of the lattice parameters and molecular structures, equations of state, densities of state for solid nitromethane is reported. By analyzing the pressure dependence of lattice parameters, a sudden change of the lattice parameters occurs between 10-12 GPa, implying that a transition has taken place. It is also found that the maximum dihedral angle of H-C-N-O has increased from 155.3° to 177.5°, indicating that a rotation of the methyl group from a staggered to an eclipsed conformation occurs in the pressure range 11–12 GPa. Before the phase transition, the intramolecular O … H–C interactions are mainly of hydrogen bonds. After the phase transition, the intramolecular and intermolecular O … H interactions are mainly of the hydrogen bonds. Phase transition also affects the reduced ratio of band gap and the density of state near the Fermi level.