A realistic dynamics simulation study is reported for the ultrafast radiationless deactivation of 9H-adenine. The simulation follows two different excitations induced by two 80 fs (fwhm) laser pulses that are different in energy: one has a photon energy of 5.0 eV, and the other has a photon energy of 4.8 eV. The simulation shows that the excited molecule decays to the electronic ground state from the (1)pi pi* state in both excitations but through two different radiationless pathways: in the 5.0 eV excitation, the decay channel involves the out-of-plane vibration of the amino group, whereas in the 4.8 eV excitation, the decay strongly associates with the deformation of the pyrimidine at the C-2 atom. The lifetime of the (1)n pi* state determined in the simulation study is 630 fs for the 5.0 eV excitation and 1120 fs for the 4.8 eV excitation. These are consistent with the experimental values of 750 and 1000 fs. We conclude that the experimentally observed difference in the lifetime of the 1n pi* state at various excitations results from the different radiationless deactivation pathways of the excited molecule to the electronic ground state.