A CASPT2/CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) under direct and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces including intermediates, transition states, conical intersections, and singlet/ triplet crossing points, for the first excited singlet (Si) and the low-lying triplet states (T-1, T-2, and T-3), We provide satisfactory explanations of many experimental findings associated with the photophysical and photochemical processes of DBO. A key finding of this work is the existence of a significantly twisted S, minimum, which can satisfactorily explain the envelope of the broad emission band of DBO. It is demonstrated that the S-1 (n-pi*) intermediate can decay to the T-1 (n-pi*) state by undergoing intersystem crossing (rather inefficient) to the T-2 (pi-pi*) state followed by internal conversion to the T, state. The high fluorescence yield and the extraordinarily long lifetime of the singlet excited DBO are due to the presence of relatively high barriers, both for intersystem crossing and for C-N cleavage. The short lifetime of the triplet DBO is caused by fast radiationless decay to the ground state.