We study vibrational energy relaxation of a "doorway model" (including the scissor,wagging and rocking mode) for nitromethane by molecular dynamics in collective interaction model.Relaxation time in vibrational cooling(VC) process agrees with experimental results.Frequency shifts for modes are consistent with experimental measurements at high-pressure in vibrational pumping(VP) process.VP process of nitromethane is observed in MD simulation in detail.It is shown that collective effects are important in vibrational energy relaxation at high-pressure and high-temperature,and energy of overtone-frequency as well as fundamental frequency of the modes must be taken into account in translational-vibrational energy transfer.