Optimized geometries and energies in the ground (S-0) and excited states of nitrobenzene were calculated using the CAS-SCF method. The optimized geometries in the S-1, S-2, T-1, T-2, and T-3 states are very different from that in the S-0 state. Most significantly, the nitro group is largely bent out of the phenyl plane in the excited states. The potential curves along the nitro rotation coordinate around the C-N bond and the out-of-plane bending mode of the nitro group in the S-0, S-1, T-1, and T-2 states were calculated in relation to the excited states dynamics. It is found that the potential curves along the bending mode in the excited states are very flat compared with that in the S-0 state. The mechanisms for the fast relaxation from the S-1 and T-1 states are discussed based on the ab initio results.